ENH: Warn on non-existent abi_migration_branches

[h-vetinari]

With the recent improvements in the validation of conda-forge.yml, I'm wondering if we could also validate that the branches as specified actually exist on the feedstock. Not doing so can be quite costly, in the sense that the bot in https://github.com/regro/cf-scripts will then retry almost every run and inevitably fail - just a lot of wasted resources for nothing IMO.

Example I came across today by chance: conda-forge/hpp-fcl-feedstock@2e0741d (the feedstock in question didn't have a v1 branch)

stacktrace from bot

From this run, but appears in every recent bot run I've looked at.

2024-04-18 00:29:19,958 INFO     conda_forge_tick.auto_tick || OSXARM-arm osx addition IS MIGRATING hpp-fcl:v1
  2024-04-18 00:29:19,958 INFO     github3 || Building a url from ('https://api.github.com/', 'rate_limit')
  2024-04-18 00:29:20,005 INFO     github3 || JSON was returned
  2024-04-18 00:29:20,006 INFO     github3 || Building a url from ('https://api.github.com/', 'user')
  2024-04-18 00:29:20,079 INFO     github3 || JSON was returned
  2024-04-18 00:29:20,080 INFO     github3 || Building a url from ('https://api.github.com/', 'repos', 'conda-forge', 'hpp-fcl-feedstock')
  2024-04-18 00:29:20,080 INFO     github3 || Missed the cache building the url
  2024-04-18 00:29:20,215 INFO     github3 || JSON was returned
  2024-04-18 00:29:20,215 INFO     github3 || Building a url from ('https://api.github.com/', 'repos', 'regro-cf-autotick-bot', 'hpp-fcl-feedstock')
  2024-04-18 00:29:20,215 INFO     github3 || Missed the cache building the url
  2024-04-18 00:29:20,338 INFO     github3 || JSON was returned
  fatal: 'upstream/v1' is not a commit and a branch 'v1' cannot be created from it
  fatal: 'upstream/v1' is not a commit and a branch 'v1' cannot be created from it
  2024-04-18 00:29:21,606 ERROR    conda_forge_tick.auto_tick || NON GITHUB ERROR
  Traceback (most recent call last):
    File "/home/runner/work/cf-scripts/cf-scripts/cf-scripts/conda_forge_tick/git_utils.py", line 260, in fetch_repo
      _run_git_cmd(f"git checkout --track upstream/{base_branch} {quiet}")
    File "/home/runner/work/cf-scripts/cf-scripts/cf-scripts/conda_forge_tick/git_utils.py", line 235, in _run_git_cmd
      return subprocess.run(cmd, shell=True, check=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "/home/runner/micromamba/envs/cf-scripts/lib/python3.11/subprocess.py", line 571, in run
      raise CalledProcessError(retcode, process.args,
  subprocess.CalledProcessError: Command 'git checkout --track upstream/v1 --quiet' returned non-zero exit status 128.
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "/home/runner/work/cf-scripts/cf-scripts/cf-scripts/conda_forge_tick/auto_tick.py", line 1253, in _run_migrator
      migrator_uid, pr_json = run(
                              ^^^^
    File "/home/runner/work/cf-scripts/cf-scripts/cf-scripts/conda_forge_tick/auto_tick.py", line 214, in run
      feedstock_dir, repo = get_repo(
                            ^^^^^^^^^
    File "/home/runner/work/cf-scripts/cf-scripts/cf-scripts/conda_forge_tick/git_utils.py", line 349, in get_repo
      if fetch_repo(
         ^^^^^^^^^^^
    File "/home/runner/work/cf-scripts/cf-scripts/cf-scripts/conda_forge_tick/git_utils.py", line 262, in fetch_repo
      _run_git_cmd(
    File "/home/runner/work/cf-scripts/cf-scripts/cf-scripts/conda_forge_tick/git_utils.py", line 235, in _run_git_cmd
      return subprocess.run(cmd, shell=True, check=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "/home/runner/micromamba/envs/cf-scripts/lib/python3.11/subprocess.py", line 571, in run
      raise CalledProcessError(retcode, process.args,
  subprocess.CalledProcessError: Command 'git checkout -b v1 upstream/v1 --quiet' returned non-zero exit status 128.
  2024-04-18 00:29:24,330 INFO     conda_forge_tick.auto_tick || /home/runner/work/cf-scripts/cf-scripts/cf-graph
  2024-04-18 00:29:24,330 INFO     github3 || Building a url from ('https://api.github.com/', 'rate_limit')
  2024-04-18 00:29:24,383 INFO     github3 || JSON was returned