gmg deleteuser doesn't fail gracefully

[ayleph]

Attempt to delete a non-existent user using bin/gmg deleteuser and you'll get a Traceback similar to below.

$ bin/gmg deleteuser idontexist
Traceback (most recent call last):
  File "bin/gmg", line 9, in <module>
    load_entry_point('mediagoblin==0.7.1.dev', 'console_scripts', 'gmg')()
  File "/path/to/mediagoblin/mediagoblin/gmg_commands/__init__.py", line 124, in main_cli
    args.func(args)
  File "/path/to/mediagoblin/mediagoblin/gmg_commands/users.py", line 132, in deleteuser
    username=unicode(args.username.lower())).one()
  File "build/bdist.linux-x86_64/egg/sqlalchemy/orm/query.py", line 2329, in one
sqlalchemy.orm.exc.NoResultFound: No row was found for one()