[Bug Fix] on-prem TFS missing collections

[ShawnMcGough]

The TfsConnection constructor allows for invalid serviceUrl when using on-prem server. Subsequent commands will report success, even though they fail to do anything.

Testing setup:
On-prem server with a serviceUrl in the following format: http://www.example.com/tfs
PAT token created at the server level ex: http://www.example.com/tfs/_usersSettings/tokens

tfx login
Service URL: {http://www.example.com/tfs}

it should fail here, because there is no collection path, but it does not

enter PAT token and run a command. for example, this one will incorrectly report success.
tfx build tasks upload --task-zip-path {path to task}

With this PR, login will correctly fail if no collection path is provided when using on-prem.a

[ShawnMcGough]

@kirill-ivlev thanks for approving! Do you know who else can approve to get this PR completed? My customer still runs into this issue occasionally and it would be great to have this in the official release.