subcommand option errors fall back to top-level usage

[HoyeonRhee]

When an unknown option is passed after a subcommand, b4 reports the error
from the top-level parser and prints the global usage.
(version: b4 0.16-dev-bc4c7)

$ b4 dig --unknown-arg

# Actual:
usage: b4 [-h] [--version] [-d] [-q] [-n] [--offline-mode] [--no-stdin]
          [-c NAME=VALUE]
          {mbox,am,shazam,review,pr,ty,diff,kr,prep,trailers,send,dig} ...
b4: error: unrecognized arguments: --unknown-arg

# Expected:
usage: b4 dig [-h] [-c COMMITISH] [-C] [-a | -m DEST | -w]
b4 dig: error: unrecognized arguments: --unknown-arg

I would expect the selected subcommand to report its own option instead.