Give more instructive error message on attempts to parse unsupported file formats

[michaelosthege]

We have a test file that is a CSV exported from the BioLection, which is not supported by bletl.parse.

As per user feedback, attempts to parse such files should give more informative error messages.

We already have a test that can be extended to match a more informative error message:

bletl/tests/test_core.py

Lines 83 to 86 in 5fc74b3

	def test_fail_on_unsupported(self):
	with pytest.raises(ValueError):
	bletl.get_parser(not_a_bl_file)
	return