Properly report pub.dartlang.org API errors.
Properly report pub.dartlang.org API errors.

There were two issues here: first, there was leftover debugging code that was
printing stack traces for all unsuccessful HTTP responses. Second, JSON-encoded
errors from the server were considered "unexpected errors", so their stack
traces were printed along with some scary text. This CL fixes both issues.

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//59223005

git-svn-id: a962b240025cf0577aa172e20f49ccb159ba6be1@30077 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/pub,dart-lang/pub,maxwellpeterson-wf/pub,cjkao/dart-pub,computmaxer/pub