Feature request: exceptions when alignment fails

[krokicki]

When insufficient moving spots are found, currently the issue is logged and the identity matrix is returned. It would be nice to throw an exception in this and other exceptional cases.

[GFleishman]

Hi @krokicki and @cgoina!

I have added this feature to the master branch. There are three relevant commits.
Here I update the docstring to describe the new functionality: 6afdb8b
Here I update the code so that failed safeguards now raise a ValueError: 0930479
Here I make sure that this behavior is turned off in the distributed context (where we want warnings and defaults instead of exceptions): d81ad7b

I'll wait a bit before closing the thread in case either of you want to look at the update or comment?