Raise better exception if authentication error occurs

[simonw]

Got this traceback (my token was invalid):

  File "...python3.9/site-packages/github_contents.py", line 51, in read_large
    tree_entry = [t for t in default_tree["tree"] if t["path"] == filepath][0]
KeyError: 'tree'

Relevant code:

github-contents/github_contents.py

Lines 44 to 53 in c4b709a

	def read_large(self, filepath):
	"Returns (file_contents_in_bytes, sha1)"
	default_tree = self.session.get(
	self.base_url() + "/git/trees/{}?recursive=1".format(self.branch),
	headers=self.headers(),
	).json()
	try:
	tree_entry = [t for t in default_tree["tree"] if t["path"] == filepath][0]
	except IndexError:
	raise self.NotFound(filepath)