Log the type of exceptions caught by pub.

We're seeing some user reports of exceptions where the type is
ambiguous.

R=rnystrom@google.com
BUG=

Review URL: https://codereview.chromium.org//149063002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@32092 260f80e4-7a28-3924-810f-c04153c831b5
1 file changed