Avoid having custom text in build-step header when running custom script.
Avoid having custom text in build-step header when running custom script.

This did not work on the build-bots.

Also don't swallow errors for custom scripts.
There should be nothing more to do after a failed custom script anyway.

BUG=
R=ricow@google.com

Review URL: https://chromiumcodereview.appspot.com//1149763006

dart-lang/package-bots