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: c93d8a2297af3b929165606efe145742a534bc71@30077 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk