Log rustfmt parsing errors as warnings

We unconditionally pass an edition parameter to rustfmt, for some crates
this might fail rustfmt so instead of swallowing the error, at least Log
it on a level that is logged by default so users won't be completely
confused about it.
See for context rust-lang#10209

Closes rust-lang#10209

crates/rust-analyzer/src/handlers.rs

@@ -1777,7 +1777,7 @@ fn run_rustfmt(
 
     let line_index = snap.file_line_index(file_id)?;
 
-    let mut rustfmt = match snap.config.rustfmt() {
+    let mut command = match snap.config.rustfmt() {
         RustfmtConfig::Rustfmt { extra_args, enable_range_formatting } => {
             let mut cmd = process::Command::new(toolchain::rustfmt());
             cmd.args(extra_args);
@@ -1842,12 +1842,12 @@ fn run_rustfmt(
         }
     };
 
-    let mut rustfmt = rustfmt
+    let mut rustfmt = command
         .stdin(Stdio::piped())
         .stdout(Stdio::piped())
         .stderr(Stdio::piped())
         .spawn()
-        .context(format!("Failed to spawn {:?}", rustfmt))?;
+        .context(format!("Failed to spawn {:?}", command))?;
 
     rustfmt.stdin.as_mut().unwrap().write_all(file.as_bytes())?;
 
@@ -1866,7 +1866,11 @@ fn run_rustfmt(
                 // formatting because otherwise an error is surfaced to the user on top of the
                 // syntax error diagnostics they're already receiving. This is especially jarring
                 // if they have format on save enabled.
-                tracing::info!("rustfmt exited with status 1, assuming parse error and ignoring");
+                tracing::warn!(
+                    ?command,
+                    %captured_stderr,
+                    "rustfmt exited with status 1"
+                );
                 Ok(None)
             }
             _ => {