Make travis script more verbose upon error (#9545)

This will help us know for sure exactly which sub-script erred.

dev/bots/docs.sh

@@ -18,4 +18,9 @@ cp dev/docs/google2ed1af765c529f57.html dev/docs/doc
 if [ "$TRAVIS_PULL_REQUEST" = "false" ] && [ "$TRAVIS_BRANCH" = "master" ]; then
   cd dev/docs
   firebase deploy --project docs-flutter-io
+  exit_code=$?
+  if [[ $exit_code -ne 0 ]]; then
+      >&2 echo "Error deploying docs via firebase ($exit_code)"
+      exit $exit_code
+  fi
 fi

dev/bots/travis_upload.sh

@@ -20,4 +20,9 @@ fi
 if [ "$SHARD" = "docs" ]; then
   # generate the API docs, upload them
   ./dev/bots/docs.sh
+  exit_code=$?
+  if [[ $exit_code -ne 0 ]]; then
+      >&2 echo "Error generating docs ($exit_code)"
+      exit $exit_code
+  fi
 fi

dev/tools/javadoc.dart

@@ -37,4 +37,5 @@ Future<Null> main(List<String> args) async {
     print('Expected file ${testFile.path} not found');
     exit(1);
   }
+  print('Javadocs ready to go!');
 }