These exceptions are almost always caused by a legitimate user
error (e.g. a nonexistent repository), and as such shouldn't be
reported as an unexpected error with the suggestion to use "pub
--trace".

R=rnystrom@google.com
BUG=20278

Review URL: https://codereview.chromium.org//425363003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@38750 260f80e4-7a28-3924-810f-c04153c831b5