generate_ansi_c_start_function reports user errors via return code

[tautschnig]

Do not throw an exception upon invalid source input. Instead, print an
error message and return true.

• 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 #5834 (2c59e0d) into develop (21a6267) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #5834   +/-   ##
========================================
  Coverage    76.10%   76.10%           
========================================
  Files         1548     1548           
  Lines       166326   166334    +8     
========================================
+ Hits        126576   126584    +8     
  Misses       39750    39750           

Impacted Files	Coverage Δ
src/ansi-c/ansi_c_entry_point.cpp	87.89% <100.00%> (+0.45%)	⬆️

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[chrisr-diffblue]

Yep, that looks more appropriate to the test of this function body.