bug: Add diagnostic information on font load error (#235)

* bug: Add diagnostic information on font load error

I ran into a bug a while back that prevent me from using the plotters
library for months. I did finally figure out what was causing it: a file
in my font library documented in issue#231. Removing the file fixed my
issue, but the opaque problem is sure to ensnare others.

This PR adds a `LoadError(Cow<'static, str>)` variant to
`SelectionError` enum. It removes `Copy` because `Cow` isn't copyable.
And now when I run the test suite with the bad file in place, it reports
this error, which is far and away more helpful than before.

```sh
> RUST_BACKTRACE=1 cargo test --all
...
running 5 tests
test loaders::core_text::test::test_core_text_to_css_font_stretch ... ok
test loaders::core_text::test::test_core_text_to_css_font_weight ... ok
test sources::core_text::test::test_css_to_core_text_font_stretch ... ok
test sources::core_text::test::test_css_to_core_text_font_weight ... ok
test loaders::core_text::test::test_from_core_graphics_font ... FAILED

failures:

---- loaders::core_text::test::test_from_core_graphics_font stdout ----
thread 'loaders::core_text::test::test_from_core_graphics_font' panicked at src/loaders/core_text.rs:940:14:
called `Result::unwrap()` on an `Err` value: LoadError("Parse error on path \"/Users/shane/Library/Fonts/Arial\"")
stack backtrace:
   0: rust_begin_unwind
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/std/src/panicking.rs:645:5
   1: core::panicking::panic_fmt
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/panicking.rs:72:14
   2: core::result::unwrap_failed
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/result.rs:1649:5
   3: core::result::Result<T,E>::unwrap
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/result.rs:1073:23
   4: font_kit::loaders::core_text::test::test_from_core_graphics_font
             at ./src/loaders/core_text.rs:938:21
   5: font_kit::loaders::core_text::test::test_from_core_graphics_font::{{closure}}
             at ./src/loaders/core_text.rs:937:38
   6: core::ops::function::FnOnce::call_once
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/ops/function.rs:250:5
   7: core::ops::function::FnOnce::call_once
             at /rustc/07dca489ac2d933c78d3c5158e3f43beefeb02ce/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

failures:
    loaders::core_text::test::test_from_core_graphics_font

test result: FAILED. 4 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.04s

error: test failed, to rerun pass `--lib`
```

A variant that used `String` would suffice for this case. I tend to use
`Cow<'static, str>` on errors since many times they can be `&'static
str`.

* refactor: Prefer CannotAccessSource to new variant

* chore: Run cargo fmt.

src/error.rs

@@ -10,6 +10,7 @@
 
 //! Various types of errors that `font-kit` can return.
 
+use std::borrow::Cow;
 use std::convert::From;
 use std::error::Error;
 use std::io;
@@ -91,18 +92,21 @@ impl From<winapi::um::winnt::HRESULT> for GlyphLoadingError {
 }
 
 /// Reasons why a source might fail to look up a font or fonts.
-#[derive(Clone, Copy, PartialEq, Debug)]
+#[derive(Clone, PartialEq, Debug)]
 pub enum SelectionError {
     /// No font matching the given query was found.
     NotFound,
     /// The source was inaccessible because of an I/O or similar error.
-    CannotAccessSource,
+    CannotAccessSource {
+        /// Additional diagnostic information may include file name
+        reason: Option<Cow<'static, str>>,
+    },
 }
 
 impl Error for SelectionError {}
 
 impl_display! { SelectionError, {
         NotFound => "no font found",
-        CannotAccessSource => "failed to access source",
+        CannotAccessSource { reason: ref maybe_cow } => maybe_cow.as_deref().unwrap_or("failed to access source")
     }
 }

src/sources/core_text.rs

@@ -230,13 +230,13 @@ fn create_handle_from_descriptor(descriptor: &CTFontDescriptor) -> Result<Handle
     let mut file = if let Ok(file) = File::open(&font_path) {
         file
     } else {
-        return Err(SelectionError::CannotAccessSource);
+        return Err(SelectionError::CannotAccessSource { reason: None });
     };
 
     let font_data = if let Ok(font_data) = utils::slurp_file(&mut file) {
         Arc::new(font_data)
     } else {
-        return Err(SelectionError::CannotAccessSource);
+        return Err(SelectionError::CannotAccessSource { reason: None });
     };
 
     match Font::analyze_bytes(Arc::clone(&font_data)) {
@@ -255,7 +255,9 @@ fn create_handle_from_descriptor(descriptor: &CTFontDescriptor) -> Result<Handle
             Err(SelectionError::NotFound)
         }
         Ok(FileType::Single) => Ok(Handle::from_memory(font_data, 0)),
-        Err(_) => Err(SelectionError::CannotAccessSource),
+        Err(e) => Err(SelectionError::CannotAccessSource {
+            reason: Some(format!("{:?} error on path {:?}", e, font_path).into()),
+        }),
     }
 }