Check for successful connection in hook_requirements()

Problem/Motivation

The hook_requirements() checks if the class can be found.

We recently had an API key invalidated and there was no error in the requirements report.
It required the forms to be submitted to discover that the API didn't work.

Proposed resolution

Try to connect in the hook_requirements() and report success or failure.

Thus, we have a standardised check point and tools like Monitoring can check for stable operations.

Remaining tasks

User interface changes

API changes

Data model changes