Handle error properly when adding more than one VCS Trigger

[cvbarros]

TeamCity returns an error if adding more than one VCS Trigger to a build configuration.
Currently, the UI hides the option to select a VCS Trigger in case an existing VCS Trigger has been added.

Possible workarounds

1. Do a GET first to verify, before the POST to create. Return a specific error in case another VCS Trigger exists.
2. POST regardless and handle TeamCity response gracefully and return error to caller.