Print the name of missing runtime procedure

This continues the fix made at 46b7abee9fdd339bac31fe7bc79551f2b3881f46,
by asserting on missing entity early and printing it's name if its not
found.

src/llvm_backend_proc.cpp

@@ -985,6 +985,7 @@ gb_internal lbValue lb_emit_call_internal(lbProcedure *p, lbValue value, lbValue
 gb_internal lbValue lb_lookup_runtime_procedure(lbModule *m, String const &name) {
 	AstPackage *pkg = m->info->runtime_package;
 	Entity *e = scope_lookup_current(pkg->scope, name);
+	GB_ASSERT_MSG(e != nullptr, "Runtime procedure not found: %s", name);
 	return lb_find_procedure_value_from_entity(m, e);
 }