Remove custom exceptions

[bradyrx]

Removes custom exceptions DimensionError and CoordinateError. This is probably bad practice for a base package like this. It makes it hard to anticipate certain errors, as encountered in https://github.com/bradyrx/climpred/pull/395. For climpred classes, I anticipate errors when looping through datasets in our objects. We should just be able to do try/except with ValueError and KeyError without importing the esmtools DimensionError.