Fix misleading error message for --generate-function-body on existing bodies

[tautschnig]

Previously, when the option was used with a function name that matched an existing function that already had a body, the error message incorrectly stated "No function name matched regex", which confused users into thinking their regex was wrong when in fact the function was found but had an existing body.

The fix improves user experience by providing clear, actionable error messages that distinguish between:

• Functions that don't match the provided regex
• Functions that match the regex but already have bodies (and thus cannot have bodies generated)

Fixes: #2974

• 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

❌ Patch coverage is 73.68421% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.94%. Comparing base (4fe3ade) to head (5a1e105).

Files with missing lines	Patch %	Lines
src/goto-instrument/generate_function_bodies.cpp	73.68%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8786   +/-   ##
========================================
  Coverage    79.93%   79.94%           
========================================
  Files         1698     1698           
  Lines       187698   187703    +5     
  Branches        73       73           
========================================
+ Hits        150034   150053   +19     
+ Misses       37664    37650   -14     

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