Return JSON response from syntax highlighter in presence of errors

[varungandhi-src]

The Rocket server we're using sometimes return HTML responses. In error cases, it should be possible to make it return JSON responses.

error: decoding JSON response from http://syntect-server:9238/: invalid character '<' looking for beginning of value

https://rocket.rs/guide/v0.5/requests/#built-in-catcher

The above docs make it sound like simply setting a header Accept: application/json should be good enough to do the trick. There is also a customization point here if needed: https://api.rocket.rs/v0.5/rocket/catcher/struct.Catcher

We should add an integration test to make sure we're returning JSON in the error case (Rocket docs on testing: https://rocket.rs/guide/v0.5/testing/). We should also make sure that the caller in gosyntect handles the error case properly (e.g. logging).

Related:

• https://github.com/sourcegraph/sourcegraph/issues/2360
• https://github.com/sourcegraph/sourcegraph/issues/59022