(Part 2) Design and integrate dedicated error handler for invalid links

[jnywong]

Context

Follow up to #7085

Proposal

1. This can go wrong if the remote repo is inaccessible, e.g. the remote repo is private, the URL is wrong, cloudflare is down, etc.
   • Let's provide a link to the git content repo for the link reader to click, so that they can see a 40x/50x error for themselves, and they can choose to copy paste error to report to the link author
2. This happens when the nbgitpuller link is malformed and points to an invalid target repository
   • e.g. git ls-remote --heads -- https://github.com/skompalli/modules-development/tree/main/Sentiment%20Analysis 👉 the extra subpath /tree/main/Sentiment%20Analysis causes the error
   • We can again provide a link to the git content repo for the link reader to click, so that they can see a 40x/50x error for themselves. We can also reiterate the expected structure of a git content repo on the error screen

Definition of Done

• Users encountering invalid links are presented with a specialised error handler that provides useful context and mitigation strategies

[jnywong]

Hey @yuvipanda, jupyterhub/nbgitpuller#385 is ready for review, and I re-requested review for the preceding PR jupyterhub/nbgitpuller#383 👍