Fix format-security violations (#802)

croak() expects printf-style format strings. Calling croak(e.what()) directly
causes compilations to fail with -Werror=format-security

xs/xsp/Print.xsp

@@ -212,7 +212,7 @@ _constant()
             try {
                 RETVAL = THIS->output_filepath(path);
             } catch (std::exception& e) {
-                croak(e.what());
+                croak("%s\n", e.what());
             }
         %};