Improve error message when glab command does not work outside of git repository directory

Problem to solve

If a repository is not supplied with -R the following error is displayed, and it doesn't inform the user how to correct their command.

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❯ glab mr list
fatal: not a git repository (or any of the parent directories): .git
git: exit status 128

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❯ glab issue list
fatal: not a git repository (or any of the parent directories): .git
git: exit status 128

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❯ GITLAB_HOST=foo glab issue list
fatal: not a git repository (or any of the parent directories): .git
git: exit status 128

Proposal

A better error would tell the user they can use -R foo/bar to specify a repository rathen than a git error.