C front-end: clean-up strings being thrown as exceptions

[tautschnig]

The remaining instances were all invariants and should not actually be
thrown as (user-facing) exceptions at all.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a 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

Merging #5836 (c581d35) into develop (316ae8b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5836   +/-   ##
========================================
  Coverage    73.56%   73.56%           
========================================
  Files         1431     1431           
  Lines       155291   155286    -5     
========================================
- Hits        114243   114242    -1     
+ Misses       41048    41044    -4     

Impacted Files	Coverage Δ
src/ansi-c/literals/convert_character_literal.cpp	71.79% <100.00%> (+3.50%)	⬆️
src/ansi-c/literals/convert_string_literal.cpp	97.05% <100.00%> (+2.69%)	⬆️

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