URL connected to nonexisting proposal leads to server error instead of 404

[tijmenbaarda]

For example, https://fetc.tst.hum.uu.nl/proposals/wmo/create/53/. In other paths (including /proposals/study_start/...) this appears to be working correctly.

Connected Sentry issue: https://uu-cdh.sentry.io/issues/10131311/?project=4508008290123856&referrer=project-issue-stream

[miggol]

This should be a pretty easy fix, but not a priority for 4.0