diag: define error_raise and diag_raise only in C++ code

We must not throw exceptions from C code.

Currently, there's the only C function that uses diag_raise() - it's
space_cache_find_xc. We move it under ifdef(__cplusplus).

Follow-up #4735

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring