Fix error handling

Created by: felixfbecker

Fixes #9411 (closed), fixes #7599 (closed)

Until #9693 and #9697 (closed) are fixed we unfortunately have no alternative than to assert error message strings.

I overhauled the "Open on Sourcegraph" logic, and decided to link to the integration docs if we can't open the options menu (in native integrations). In the case where the repo is simply not found (but not because it is private + instance is sourcegraph.com), I show that in the button but still link to the repo on the instance, where it will get the 404 page that could have more instructions or a button to configure external services depending on the site admin status of the user etc.

Example: