Add handling covering incr/3 on non-numeric keys

[whitfin]

Currently there's a bug in decr/3 and incr/3 in that if you create a non-numeric value beforehand, a call to either will crash the server.

This PR will add coverage around this issue and resolve it in all workers. I also added handling to get_and_update/4 to better support transactions and unrecognised errors.

You can now Cachex.abort(worker, :reason) from inside a get_and_update/4 call to exit early and not commit the changes, and { :error, :reason } will be returned.

[coveralls]

Coverage remained the same at 100.0% when pulling 4b92160 on non-numeric-incr into 2d8353b on master.