Add type checking and raise meaningful exceptions

[albcui]

Some of our client methods accept various types for inputs, for example in get_file(commit, path, URL), the commit can be a tuple, string, or Commit object. While we do some type checking with

python-pachyderm/src/python_pachyderm/mixin/util.py

Line 4 in 8b5027d

def commit_from(src, allow_just_repo=False):

, but this might break for users that specified an invalid string, or a string that got the order of repo and commit wrong.

In general, we want to add type checking where possible, and raise meaningful exceptions when inputs are not valid.