remote.nextstrain_dot_org: Handle 400 Bad Request errors

[tsibley]

This helps in particular with user-caused bad requests, such as trying to upload to dataset URLs which include underscores instead of slashes. It will also let us use 400 responses more widely on the server side since we know the CLI user will now see the error message.

Related-to: nextstrain/nextstrain.org#611

Testing

Example of the error message:

$ NEXTSTRAIN_DOT_ORG=http://localhost:5000 nextstrain remote upload groups/test/foo_bar foo_bar
.json
Uploading foo_bar.json as http://localhost:5000/groups/test/foo_bar
Error: The remote server rejected our request:

  Resource (e.g. dataset and narrative) paths may not include
  underscores (_); use slashes (/) instead

This may indicate a problem that's fixable by you, or it
may be a bug somewhere that needs to be fixed by the
Nextstrain developers.

If you're unable to address the problem, please open a new
issue at <https://github.com/nextstrain/cli/issues/new/choose>
and include the complete output above and the command you
were running.

• Checks pass
• Manual testing of 400 with decodable JSON body
• Manual testing of 400 with bad JSON body

[tsibley]

Ugh, CI is failing (again) for unrelated reasons (again). This time, it appears to be a recent release of flake8 with more stringent config parsing. Will fix on another PR (again).

[tsibley]

Rebased to fix CI issues.