CrabPascal Stops Faking: Exception Handling Now Explicitly Fails

Sprint 13 solves a critical trust problem: the C backend now explicitly fails on unsupported exception handling instead of silently generating non-functional code that compiles but crashes at runtime. This prevents developers from unknowingly shipping broken builds, addressing the false positive that plagued previous releases. The change reflects CrabPascal's core principle of honest tooling—clear failure beats silent stubs.