introduce << on cprover_exception_baset

[kroening]

This introduces the operator << on error_exceptiont, which offers the option to extend the reason message. Any exception derived from cprover_exception_baset can then be thrown as follows:

  throw some_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

❌ Patch coverage is 97.36842% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.77%. Comparing base (2a9730d) to head (4720ee6).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/goto-programs/restrict_function_pointers.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #9104   +/-   ##
========================================
  Coverage    80.77%   80.77%           
========================================
  Files         1712     1713    +1     
  Lines       189814   189838   +24     
  Branches        73       73           
========================================
+ Hits        153328   153348   +20     
- Misses       36486    36490    +4     

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