Improved error message for branch --init when branch doesn't exist

Fix of #425.
- Removed magic value -1 returned from GetRootChangesetForBranch. Now throws instead.
- Updated calling code and a test case accordingly.

Old output:
The name of the local branch will be : asdf
error: No root changeset found :(

New output:
The name of the local branch will be : asdf
error: TFS branches $/MyProject/asdf not found!