Plugins should throw exceptions for startup errors

Same rationale as #273 (closed).

Let's work on this as part of #136 (closed).

Added ~15 label

Milestone changed to %Milestone F

assigned to @grote

created branch 285-plugins-should-throw-exceptions-for-startup-errors

added ~8 label

mentioned in merge request !462 (merged)

added ~11 and removed ~8 labels

closed via commit e7a26c42

added ~10 and removed ~11 ~15 labels

mentioned in commit e7a26c42