Add ERRORK opcode to trigger constant errors

src/compile.h

@@ -19,6 +19,7 @@ block gen_location(location, struct locfile*, block);
 block gen_noop();
 int block_is_noop(block b);
 block gen_op_simple(opcode op);
+block gen_error(jv constant);
 block gen_const(jv constant);
 block gen_const_global(jv constant, const char *name);
 int block_is_const(block b);