Better error reporting for when a repo can't be found

[dwilding]

For example, I don't have a fork of https://github.com/canonical/multipass. If I run gimmegit multipass, gimmegit tries to clone https://github.com/dwilding/multipass (because I have GIMMEGIT_GITHUB_TOKEN set), but there's no such repo. gimmegit crashes:

Getting repo details
Traceback (most recent call last):
  File "/home/david.wilding@canonical.com/.local/bin/gimmegit", line 10, in <module>
    sys.exit(main())
             ~~~~^^
  File "/home/david.wilding@canonical.com/.local/share/uv/tools/gimmegit/lib/python3.13/site-packages/gimmegit/_cli.py", line 86, in main
    context = get_context(args)
  File "/home/david.wilding@canonical.com/.local/share/uv/tools/gimmegit/lib/python3.13/site-packages/gimmegit/_cli.py", line 169, in get_context
    upstream = get_github_upstream(owner, project)
  File "/home/david.wilding@canonical.com/.local/share/uv/tools/gimmegit/lib/python3.13/site-packages/gimmegit/_cli.py", line 243, in get_github_upstream
    repo = api.get_repo(f"{owner}/{project}")
  File "/home/david.wilding@canonical.com/.local/share/uv/tools/gimmegit/lib/python3.13/site-packages/github/MainClass.py", line 473, in get_repo
    headers, data = self.__requester.requestJsonAndCheck("GET", url)
                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/home/david.wilding@canonical.com/.local/share/uv/tools/gimmegit/lib/python3.13/site-packages/github/Requester.py", line 622, in requestJsonAndCheck
    return self.__check(
           ~~~~~~~~~~~~^
        *self.requestJson(
        ^^^^^^^^^^^^^^^^^^
    ...<7 lines>...
        )
        ^
    )
    ^
  File "/home/david.wilding@canonical.com/.local/share/uv/tools/gimmegit/lib/python3.13/site-packages/github/Requester.py", line 790, in __check
    raise self.createException(status, responseHeaders, data)
github.GithubException.UnknownObjectException: 404 {"message": "Not Found", "documentation_url": "https://docs.github.com/rest/repos/repos#get-a-repository", "status": "404"}

gimmegit should catch this and log a useful error message.

Ideally, gimmegit should list any clones that I already have. I might have cloned canonical/multipass before, in which case I need reminding to run gimmegit canonical/multipass instead of gimmegit multipass.