Require HTTP 200 before parsing JSON, send back better error for HTTP 502, etc...

[karendolan]

When Canvas is down, CACCL apps throw error

Error: Failed to parse response as undefined: Unexpected token '<', "<!DOCTYPE "... is not valid JSON

from call at https://github.com/harvard-edtech/caccl-api/blob/main/src/shared/genVisitEndpoint/index.ts#L153

This appears to users as message

This app cannot be launched this way because it is not yet installed into the course that you are launching from. Please contact support.

It would be less confusing to show a message that the CACCL app cannot be launched because of Communication problems with Canvas (for example, during a HTTP 502 response) and also to recommend that the user try to access the application directly from the Canvas course site. Which should resolve most problems anyway.

[karendolan]

Also, in general, it would be very helpful to send back the HTTP response in the error message to get a better idea of why the JSON parse failed with the HTTP text error response body.