Detect matrix errors and raise them as exceptions

[Cadair]

Currently if the http request succeeds then we assume the matrix request also succeeded, if an error response is provided from the matrix server we should convert it to a Python exception.

octoprint-matrix-notifier-plugin/octoprint_matrix_notifier/matrix.py

Lines 45 to 46 in d748108

	# TODO: Detect matrix errors here
	return json.loads(resp.read())