Throw incorrect/missing gapi config properties when gapi determines the properties are incorrect

[begedin]

Currently, this is what happens when we set a google API key or other property in the configuration, but we set it incorrectly:

Google's pop-up registers an error, but there's no helpful error message from the add-on, like it is when the keys are missing.

The same error that get's thrown when a key is missing should also be thrown when the key is present, but it's determined to be invalid/incorrect. This way, the user will know what's going on even in cases like these.

Hopefully, Google's pop-up somehow returns an error as a result when this happens, so we should be able to get hold of that error.

[venkatd]

This makes sense. The priority would be missing or empty keys. If raising an invalid key error turns out to be too much work we can skip over it.