Require explicit contract for --replace-call-with-contract

[tautschnig]

Change --replace-call-with-contract to produce a hard error instead of a warning when used with functions lacking explicit contracts. This change addresses a soundness issue where CBMC would previously assume a trivial contract automatically.

Users that really need a trivial contract with no constraints should use

int my_function(int x)
  __CPROVER_requires(1)
  __CPROVER_ensures(1)
{
  return x + 1;
}

Fixes: #8728

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• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• 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 57.14286% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.94%. Comparing base (0985044) to head (2a32be7).

Files with missing lines	Patch %	Lines
...contracts/dynamic-frames/dfcc_contract_handler.cpp	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8739      +/-   ##
===========================================
- Coverage    79.94%   79.94%   -0.01%     
===========================================
  Files         1699     1699              
  Lines       187790   187786       -4     
  Branches        73       73              
===========================================
- Hits        150133   150122      -11     
- Misses       37657    37664       +7     

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