gclient setdep: Command line interface nits.
gclient setdep: Command line interface nits.

'setdep' now shows as a command when typing 'gclient help'.
We now use parser errors to indicate invalid formatting of flags, and
raise an error when there are unused arguments.

R=bbed39beedae4cdb499af742d2fcd6c63934d93b@chromium.org

Bug: 760633
Change-Id: I37fa65ef0c2cff9de1234efcf22bcd888a5e9589
Reviewed-on: https://chromium-review.googlesource.com/998741
Commit-Queue: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>
Reviewed-by: Michael Moss <9cc3612fe50cad025435ae878219b6203d6b641d@chromium.org>
Reviewed-by: Aaron Gable <bbed39beedae4cdb499af742d2fcd6c63934d93b@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools