Return a clear error when HTTP calls to legacy repo fail, instead of PackageNotFound

[MarcDufresne]

This PR aims to make Poetry a bit more clear about issues when communicating with a legacy repo.

For example, it happened a few times that we would get a PackageNotFound error when trying to install a package from a private repo, when in fact the actual issue was bad credentials. The simple fix is simply to return the HTTP error message instead of a generic PackageNotFound.

With this change you will now see:

[RepositoryError]                                                              
401 Client Error: Unauthorized for url: https://my-repo.com/repo/my-package/

Instead of:

[PackageNotFound]        
Package [my-package] not found. 

[sdispater]

Can you add a test covering this?

[MarcDufresne]

Done! I was hesitant to add tests for _get and _download since they were not already tested and didn't want to take design decisions regarding testing.
However since you asked for them, I took the liberty of adding requests-mock and test both happy and sad path for both methods, let me know what you think.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

Closing this pull request automatically because it has not had any activity since it has been marked as stale. If you think it is still relevant and should be addressed, feel free to open a new one.

[dan-hipschman]

Any objection to reopening and merging this? I was about to make a PR for the same improvement, but this one seems perfectly good. Thanks!

[finswimmer]

Closing this in favor of #2459

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.