Improve error message when src directory for repo add doesn't exist

[punchagan]

Closes #5632

[rjbou]

Thanks for the PR. On the general idea, lgtm!
I'll add a test to it, in order to complete it.

[punchagan]

I'll add a test to it, in order to complete it.

There are some existing tests which already fail because of this change. I will go ahead and fix them, to begin with.

[rjbou]

I've added a commit that contains the code review & test update.

[punchagan]

Thanks for the code improvements and the test fixes, @rjbou !

[punchagan]

Thanks for the code improvements and the test fixes, @rjbou !

Let me know if I need to do anything more to finish this PR, @rjbou ! Thanks!

[rjbou]

I've updated the PR, a review from @kit-ty-kate and it's good to merge!

[rjbou]

Sorry for the delay, we are focusing on the release, and there is only me and partly kate working on opam.

[punchagan]

Sorry for the delay, we are focusing on the release, and there is only me and partly kate working on opam.

Thank you for your work on opam!

No worries at all -- I'm not in a hurry to have this merged. I just didn't want to leave it hanging there, and wanted clarity on whether it's waiting on me.

[kit-ty-kate]

Compared to #6027 I think the errors are less readable. A good error message already exist in the data (the second value of the pair) and duplicating it seems superfluous.