Gracefully handle IO errors when failing to overwrite a file.

lib/src/formatter_options.dart

Index: lib/src/formatter_options.dart
diff --git a/lib/src/formatter_options.dart b/lib/src/formatter_options.dart
index 248941fb9ae088a2e19af01d14687217e6a2aa06..6195d189ad7f294afbe8b1d8ff7894e965cf6861 100644
--- a/lib/src/formatter_options.dart
+++ b/lib/src/formatter_options.dart
@@ -116,8 +116,13 @@ class _PrintJsonReporter extends OutputReporter {
 class _OverwriteReporter extends _PrintReporter {
   void afterFile(File file, String label, SourceCode output, {bool changed}) {
     if (changed) {
-      file.writeAsStringSync(output.text);
-      print("Formatted $label");
+      try {
+        file.writeAsStringSync(output.text);
+        print("Formatted $label");
+      } on FileSystemException catch (err) {
+        stderr.writeln("Could not overwrite $label: "
+            "${err.osError.message} (error code ${err.osError.errorCode})");
+      }
     } else {
       print("Unchanged $label");
     }