Provide a better description of any exception that occurs. Minimum Py to 3.7

Provide a better description of any exception that occurs during a subscription, if the JAPC provided description is empty.

Came up in discussion today (internal link: https://mattermost.web.cern.ch/acc-py/pl/faqjqdqjej8kbrmtpqaut46m5c). Essentially:

2022-01-26 16:52:52 - [ERROR] - pyjapc - Parameter BA1.VATOMO/Targets received exception:

When I digged deeper, I could find out:

cern.japc.core.ParameterException: None LSA context is mapped to PSB.USER.MD8