Web UI reports "Oops! Internal Server Error" when job hasn't started yet

[talex5]

(originally reported at https://github.com/ocurrent/solver-service/issues/74#issuecomment-1751679323 by @patricoferris)

The web UI is currently showing "500 Oops! Internal Server Error" for lots of jobs (example). Looking at the logs, it seems that it got a decent error message from ocaml-ci (No job for variant "(analysis)" yet):

07.10.23 11:14:36.284       dream.log  INFO REQ 68411 GET /github/ocaml-multicore/eio/commit/69c622fa0184606164bfbb6a257768c87d2a9d29/variant/%28analysis%29 10.0.1.4:55160 Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0
07.10.23 11:14:36.330                       REQ 68411 Internal server error: Failed: No job for variant "(analysis)" yet

It should display the reason for the problem to the user instead of an internal server error in this case.