Custom message type for compiletime.error

[yurikpanic]

Fixes summonInline behavior in #15788

The fix is perhaps of a "hacky" kind. But it fixes the original issue described in #15788 and does not break any existing test. Perhaps we need more generic way to distinguish error cases that mean "given instance not found" from "it was found but we have some error that should be reported to the user".

[odersky]

This should be for @nicolasstucki (or anybody else who volunteers to cover this) to review. But I am pretty sure that a compiletime.error should mean that the candidate fails to typecheck, and I don't like the exceptions in the code.

The fact that this PR does not break any tests means nothing. Error diagnostics is severly under-tested.