Unexpected response from JWK key set URL causes very confusing error

[robertknight]

In this Slack thread launching the Hypothesis LMS assignment picker failed with a "could not connect to lms.hypothes.is" error.

After debugging (see thread), it turned out that the JWK key set URL had been misconfigured. The incorrect URL was returning a 3xx HTML response instead of JSON. This was handled very badly by the LMS app. Instead of triggering a helpful error indicating what happened, the app experienced an exception while attempting to parse the response from the key set URL request as JSON and this somehow led to a 502 gateway timeout. The gateway timeout HTTP response included an X-Frame-Options: SAMEORIGIN header which caused the browser to show a "failed to connect to lms.hypothes.is" error since the assignment picker is loaded in an iframe.