Return 403 and 405 from observatory server
Return 403 and 405 from observatory server

With this change untrusted `origin` header or method not allowed will
return 403 and 405 respectively. Before this change it returned 200 with
an empty body. Returning proper error codes helps users debug their
setup and figure out what is wrong.

Closes https://github.com/dart-lang/sdk/issues/34121

Change-Id: I60c75ee2e33882f98fd9a020d33d459e74febf29
Reviewed-on: https://dart-review.googlesource.com/c/69308
Commit-Queue: Jonas Jensen <c9327fa216ae2ef84df7c55b809d4ba270141b48@google.com>
Reviewed-by: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>

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