Starting in python mode should fail gracefully if pyimagej is not installed

I pointed Fiji to a custom environment that did not have pyimagej installed as I wasn't sure if it was required ahead of time or not. This led to me being unable to start Fiji.

It would be nice if I got a notification at startup that pyimagej wasn't found and we fell back to start in java mode.

Potentially we could even validate this when setting the environment.