improve invalid output error messages, fixes #106

Author: jakemac53

lib/src/asset/exceptions.dart

@@ -24,12 +24,12 @@ class PackageNotFoundException implements Exception {
 
 class InvalidOutputException implements Exception {
   final Asset asset;
+  final String message;
 
-  InvalidOutputException(this.asset);
+  InvalidOutputException(this.asset, this.message);
 
   @override
-  String toString() => 'InvalidOutputException: $asset\n'
-      'Files may only be output in the root (application) package.';
+  String toString() => 'InvalidOutputException: $asset\n$message';
 }
 
 class InvalidInputException implements Exception {

lib/src/builder/build_step_impl.dart

@@ -128,7 +128,8 @@ class BuildStepImpl implements BuildStep {
   /// [InvalidOutputException] or [UnexpectedOutputException] if it's not.
   void _checkOutput(Asset asset) {
     if (asset.id.package != _rootPackage) {
-      throw new InvalidOutputException(asset);
+      throw new InvalidOutputException(
+          asset, 'Files may only be output in the root (application) package.');
     }
     if (!expectedOutputs.any((id) => id == asset.id)) {
       throw new UnexpectedOutputException(asset);

lib/src/generate/build_impl.dart

@@ -517,10 +517,12 @@ class BuildImpl {
       /// Validate [expectedOutputs].
       for (var output in expectedOutputs) {
         if (output.package != _packageGraph.root.name) {
-          throw new InvalidOutputException(new Asset(output, ''));
+          throw new InvalidOutputException(new Asset(output, ''),
+              'Files may only be output in the root (application) package.');
         }
         if (_inputsByPackage[output.package]?.contains(output) == true) {
-          throw new InvalidOutputException(new Asset(output, ''));
+          throw new InvalidOutputException(new Asset(output, ''),
+              'Cannot overwrite inputs.');
         }
       }