Fix an error-swallowing bug in pub and several bugs it was hiding.

We were passing an error to [completer.completer] rather than
[completer.completeError] and this was hiding several behavioral bugs.

R=rnystrom@google.com
TBR

Review URL: https://codereview.chromium.org//598883002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@40621 260f80e4-7a28-3924-810f-c04153c831b5