`tagman`: Add check to verify a repo status

[yaswant]

At the moment, tagman can't resolve a sha, tag, release, or branch name when the user specifies a repo name via the -R flag that doesn't exist or can't be accessed.
The error message can be misleading; for example, 992191da90f4378c6335da2f9a4e098c3f367f0e exists in MetOffice/git_playground, but the user had a typo, like:

tagman add 'tag-name' 992191da90f4378c6335da2f9a4e098c3f367f0e -R MetOffice/git_playgroundy

** Reference '992191da90f4378c6335da2f9a4e098c3f367f0e' not found in repository 'MetOffice/git_playgroundy'.
** Tried: commit SHA, tag, release, and branch name.

There should be a check to validate repo access and fail-fast.