core: refactor trigger_fiber_run

Make trigger_fiber_run return an error, when it occurs, so that the
calling code decides how to log it.
Also, while I'm at it, simplify trigger_fiber_run's code a bit.

In-scope-of #4264

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit ca59d305)