Error handling in .load

[DAndrewA]

In the .load sub-package, no error handling is implemented in the load_fromglob or load_fromlist functions.

Might be worth implementing error handling in the cases:

1. files passed to load_fromlist directly contain files that aren't of the .mpl.gz format
2. files loaded from load_fromlist can't be concatenated by xr.combine_nested() (dimension mismatch, duplicate time-stamps, etc)