Running a multi-command like rbt review with no subcommand crashed

with a TypeError. The subparsers were not marked as required, so argparse

accepted the empty command line and left options.args as None.
Multi-commands now print their help text and exit with status 1 when run

without any arguments, similar to git. The subparsers are also marked

as required, so other invalid invocations (such as rbt review --foo)

show a usage error instead of crashing. They use a <subcommand> metavar

to keep the error and help output readable.

Ran unit tests.
Ran rbt review and saw useful help output instead of a crash.