Maestro logs that VSTS build queueing was successful even when sign-in failed

[naamunds]

Steps to reproduce the issue

1. Run the Maestro WebAPI app without valid VSO credentials (e.g. with the VsoUser and VsoPassword app settings empty).
2. Send an HTTP request to Maestro that represents a commit-pushed event which triggers an action in subscriptions.json and thus attempts to queue a build.

Expected behavior

Maestro logs a message saying that the build was not successfully queued due to invalid credentials.

Actual behavior

Maestro logs "Successfully queued VSO build."

[naamunds]

The Maestro code currently checks that the HTTP response from VSO has a success status code; in the case where the credentials were invalid, the HTTP response has a 203 status (which is considered successful), and its content appears to be a sign-in page.

A possible fix is to parse the content and look for “Sign In” or some other indicative string. Or, perhaps the 203 status code is specific to the invalid credential case, but I’m not sure we should assume that without further investigation.