400 error when merge proposal already registered

[Bryce Harrington]

Steps to reproduce:

  1. File an MP via the submit subcommand, e.g.:

     git ubuntu submit \
        --reviewer canonical-server \
        --reviewer canonical-server-reporter \
        --reviewer canonical-server-core-reviewers \
        --reviewer canonical-server-packageset-reviewers \
        --target-branch "debian/sid"

  2. Repeat same command as in step #1

  3. Full HTTP 400 error response is printed:

05/09/2023 00:17:20 - ERROR:Unable to create merge proposal: HTTP Error 400: Bad Request
Response headers:
---
-content-encoding: gzip
connection: close
content-length: 177
content-security-policy: frame-ancestors 'self';
content-type: text/plain;charset=utf-8
date: Tue, 09 May 2023 07:17:20 GMT
server: gunicorn
status: 400
strict-transport-security: max-age=15552000
vary: Accept,Accept-Encoding
x-content-type-options: nosniff
x-frame-options: SAMEORIGIN
x-launchpad-revision: 90e2150bef31b411d7bae5c4032c7e320fcaaec8
x-lazr-notifications: []
x-powered-by: Zope (www.zope.org), Python (www.python.org)
x-request-id: 6b878a91-70d3-4c25-9c35-4e76f5cd475b
x-vcs-revision: 90e2150bef31b411d7bae5c4032c7e320fcaaec8
x-xss-protection: 1; mode=block
---
Response body:
---
b'There is already a branch merge proposal registered for branch ~bryce/ubuntu/+source/chrony:merge-v4.3-2-mantic to land on ubuntu/+source/chrony:debian/sid that is still active.'
---

  4. Try again with the --force option, but it just prints the same.

I'm guessing this is failing in the createMergeProposal() call, however that appears to be handling the BadRequest exception already:

    # create MP
    try:
        mp = source_git_ref.createMergeProposal(merge_target=target_git_ref)
    except BadRequest as e:
        logging.error("Unable to create merge proposal: %s", e)
        return 1

This looks to me like it *should* handle the 400 error exception and only print the error message itself.

An obvious improvement would be to have it suggest using --force to overwrite the old MP, and then when --force is specified, to detect and remove any existing MP before creating this one.