* Clean up help text * Fix confusing error when nonexistent source_set provided.
* Clean up help text
* Fix confusing error when nonexistent source_set provided.

dbarobin/google-mysql-tools,sdgdsffdsfff/google-mysql-tools,vincentor/google-mysql-tools,dbarobin/google-mysql-tools,vincentor/google-mysql-tools,sdgdsffdsfff/google-mysql-tools