available_readers to detail unavailable items

[mraspaud]

Is your feature request related to a problem? Please describe.
As a debugging feature, it would be really helpful to have available_readers detail the reason why the unavailable reader aren't available.

Describe the solution you'd like
A possible solution would be to print out the error happening upon attempting to load the unavailable readers.

Describe any changes to existing user workflow
As this is a purely for debugging purposes, no changes in the workflow will happen.

Additional python or other dependencies
No additional dependencies

Describe any changes required to the build process
No changes to the build process

Describe alternatives you've considered
No alternatives.

Additional context
No context.

[djhoese]

Would this be a keyword argument? Would the errors be put in to debug/error log messages or in the returned value somehow? If log messages then I suppose it could always log the reasons?

[mraspaud]

I could be a kw arg that would print a report on screen, while still returning the list as expected.

[djhoese]

This has been fixed by @mraspaud by adding the check_satpy utility function.