Mod. Use SartorisError to flag exceptional behaviour in start & abort.

Change-Id: I4717229ef47b469fdf3f656ca2a610f93f09760d
Add. Return with exit code from SartorisError.

Change-Id: If281b3fa708159f29e5f60f66f05096c80f46603
Mod. Fix control on subprocess returncodes. Make commands clearer.

Change-Id: Id8c876116b0222b5545183f8fd9a8cbd6b8af716
Add. Diff functionality. helper method _get_commit_sha_for_tag().

Change-Id: I8c5715ac6119afb0b2dc10b35271272e21d5ad25