Return a more verbose error when formatting a file fails

Expands the error message returned if the after_file function fails to
also include path_str. This allows users to better identify files that are
not being formatted.

src/lib.rs

@@ -325,7 +325,14 @@ fn format_ast<F>(
         }
         visitor.format_separate_mod(module, &*filemap);
 
-        has_diff |= after_file(path_str, &mut visitor.buffer)?;
+        has_diff |= match after_file(path_str, &mut visitor.buffer) {
+            Ok(result) => result,
+            Err(e) => {
+                // Create a new error with path_str to help users see which files failed
+                let mut err_msg = path_str.to_string() + &": ".to_string() + &e.to_string();
+                return Err(io::Error::new(e.kind(), err_msg));
+            }
+        };
 
         result.push((path_str.to_owned(), visitor.buffer));
     }