Add type checking and raise meaningful exceptions

[albscui]

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 https://github.com/pachyderm/python-pachyderm/blob/8b5027dd2fd8a2a0c1b68a6ab0179a9c1d40a679/src/python_pachyderm/mixin/util.py#L4, 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.