Add a virtual destructor to cprover_exception_baset

[hannes-steffenhagen-diffblue]

While exceptions under normal usage scenarios don't need a virtual destructor
(when an exception is thrown, it is the runtimes responsibility to properly
destroy it, and it does know the type of the exception at the point where it's
thrown), it is just generally good practice to have virtual destructors in base
classes to accomodate for situations where a polymorphic destruction may be
needed.

This also adds an explanation for why we use this class in the first place instead of just deriving from std::exception

• Each commit message has a non-empty body, explaining why the change was made.
• My contribution is formatted in line with CODING_STANDARD.md.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 483f07c).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/87493058