[Isabelle] Handle internal errors more gracefully

[wimmers]

Internal errors such as

Where | What
-- | --
Internal error | An error occured while processing Isabelle Server's output (Expecting value: line 1 column 1 (char 0))

should not make it to the frontend.
Rather, the REST API endpoint should filter them out and keep the submission pending.
But we definitely need some internal singalling of these problems (e-mail?).