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:

https://github.com/notmatthancock/pylidc/blob/20beb61b8def1b417813a5b41155a55a8f0f7f5d/pylidc/Scan.py#L209-L215