Show unrecognized arguments error using subparser

[victorlin]

Description of proposed changes

This PR contains 1 prep commit + 1 main commit. Message from main commit:

Standard argparse behavior always uses the top-level parser for unrecognized arguments, rendering generic error messages.

Call parse_known_args() ourselves to access the unrecognized arguments, and make the subparser accessible as an attribute on the on the parsed arguments namespace to use it for errors.

An alternative approach is to subclass _SubParsersAction, but this requires manually injecting the custom action class into every nested parser (curate, export, titers, etc.). The current approach cleanly supports nested parsers.

Related issue(s)

Closes #2028

Checklist

• Automated checks pass
• Check if you need to add a changelog message
• Check if you need to add tests tested manually. I don't think this needs any command-specific functional test as argparse output may change over time.
• Check if you need to update docs
• Post-merge: consider applying to Nextstrain CLI too

[codecov]

Codecov Report

❌ Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.25%. Comparing base (c2768dd) to head (fc95052).

Files with missing lines	Patch %	Lines
augur/__init__.py	37.50%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2029      +/-   ##
==========================================
- Coverage   73.27%   73.25%   -0.02%     
==========================================
  Files          86       86              
  Lines       10748    10755       +7     
  Branches     2095     2096       +1     
==========================================
+ Hits         7876     7879       +3     
- Misses       2498     2502       +4     
  Partials      374      374              

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[joverlee521]

Nice! I always thought it was weird that the error didn't point to the subcommand.