Proper error message for init command

[mingrammer]

This is not a bug, but we should improve it.

When I have already migrations directory and run flask db init, it raises an exception with Python exception traceback (alembic.util.exc.CommandError: Directory migrations already exists)

Yes it is correct. But, this error message just seems like a bug of flask tool. So I think it is better that flask db init should have a proper error message instead of raises exception for this situation.