Added error handling and a better error message if we fail to write output in docgen.

pkg/docgen/lib/src/generator.dart

Index: pkg/docgen/lib/src/generator.dart
diff --git a/pkg/docgen/lib/src/generator.dart b/pkg/docgen/lib/src/generator.dart
index aba05514e1376b6bf71c8a944fd8ac1621624250..0531869abdfdc8159e716ef3f0877acb27e5583a 100644
--- a/pkg/docgen/lib/src/generator.dart
+++ b/pkg/docgen/lib/src/generator.dart
@@ -136,8 +136,15 @@ void _writeToFile(String text, String filename) {
   var parentDir = new Directory(path.dirname(filePath));
   if (!parentDir.existsSync()) parentDir.createSync(recursive: true);
 
-  new File(filePath)
-      .writeAsStringSync(text, mode: FileMode.WRITE);
+  try {
+    new File(filePath)
+        .writeAsStringSync(text, mode: FileMode.WRITE);
+  } on FileSystemException catch (e) {
+    print('Failed to write to the path $filePath. Do you have write '
+        'permissions to that directory? If not, please specify a different '
+        'output directory using the --out option.');
+    exit(1);
+  }
 }
 
 /// Resolve all the links in the introductory comments for a given library or