Refactor into try_test + syn::Result

syn::Result allows us to return spanned errors which can be converted
into nice compile errors when there is unexpected input.

For example:
```
error: Failed to parse value, expected a string!
  --> tests/mod.rs:94:33
   |
94 | #[test_log(default_log_filter = 6)]
   |                                 ^
```

CHANGELOG.md

@@ -1,3 +1,8 @@
+Unreleased
+----------
+- Improved compile error output on wrong usage
+
+
 0.2.13
 ------
 - Improved interaction with nested attributes (such as those used by the

src/lib.rs

@@ -72,12 +72,19 @@ use syn::ItemFn;
 /// ```
 #[proc_macro_attribute]
 pub fn test(attr: TokenStream, item: TokenStream) -> TokenStream {
+  let item = parse_macro_input!(item as ItemFn);
+  try_test(attr, item)
+    .unwrap_or_else(syn::Error::into_compile_error)
+    .into()
+}
+
+
+fn try_test(attr: TokenStream, input: ItemFn) -> syn::Result<Tokens> {
   let inner_test = if attr.is_empty() {
     quote! { ::core::prelude::v1::test }
   } else {
     attr.into()
   };
-  let input = parse_macro_input!(item as ItemFn);
 
   let ItemFn {
     attrs,
@@ -115,7 +122,7 @@ pub fn test(attr: TokenStream, item: TokenStream) -> TokenStream {
       #block
     }
   };
-  result.into()
+  Ok(result)
 }