Include path in error message when dicom path from config file doesn't exist

[notmatthancock]

If a user specifies a non-existent DICOM path in the pylidc config file, an error message is shown. This error message would be more helpful if it included the DICOM path that cannot be located from the config file:

pylidc/pylidc/Scan.py

Lines 209 to 215 in 20beb61

	dicompath = _get_dicom_file_path_from_config_file()
	if not os.path.exists(dicompath):
	msg = ("Could not establish path to dicom files. Have you "
	"specified the `path` option in the configuration "
	"file {}?")
	raise RuntimeError(msg.format(_get_config_file()))