introduce error_exceptiont

[kroening]

This introduces error_exceptiont, which offers an exception class with optional source_locationt and optional exit code.

The error message of the exception can be extended with a stream operator, and is typically thrown as follows:

throw error_exceptiont{} << "text" << ....;

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.78%. Comparing base (1e835df) to head (21d6b86).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #9104   +/-   ##
========================================
  Coverage    80.77%   80.78%           
========================================
  Files         1712     1713    +1     
  Lines       189814   189876   +62     
  Branches        73       73           
========================================
+ Hits        153324   153384   +60     
- Misses       36490    36492    +2     

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