Refactor Error Handling

Replace @__stk_u_throw_error with @__stk_u_userspace (boolean).
@__stk_u_userspace will be changed in procedure_call (rename it to user_procedure_call), in exceptions routines and in assert.

Remove handler for SQLWARNING, SQLEXCEPTION from test_case_run.
In user_procedure_call, move EXECUTE into a BEGIN..ENG with a HANDLER for all errors.