Improve handling of incorrect external hooks definitions

Current situation

When a provider plugin defines hooks in its configuration file, errors in the hooks section cause the plugin to fail to start. This is expected, and an orchestrator administrator can detect and fix the problem (it's easy to detect because the plugin fails to start :) ).

If a provider plugin is configured to use an external hooks definition, this external definition replaces the one possibly present in its configuration file. This is also the expected behaviour.

But if this external hooks definition is invalid (or if, at a later time, it becomes invalid), there are entries in the orchestrator logs, but this may be missed. More importantly, a user has no way to know those external hooks definition are not used.

Desired outcome

If a workflow uses a function that comes from this provider plugin, there should be a (non-blocking) error message in the workflow execution log.