Refactor entry point validation towards symex-callers

[tautschnig]

We can detect the absence of an entry point much earlier and do not need to perform unnecessary work before inevitably failing.

Fixes: #1847

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• 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

❌ Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 79.94%. Comparing base (0985044) to head (c0d7491).

Files with missing lines	Patch %	Lines
...-checker/single_loop_incremental_symex_checker.cpp	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8782   +/-   ##
========================================
  Coverage    79.94%   79.94%           
========================================
  Files         1699     1699           
  Lines       187790   187801   +11     
  Branches        73       73           
========================================
+ Hits        150133   150143   +10     
- Misses       37657    37658    +1     

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