-/* If "err" != 0, exit with an error message "msg" and clean up. If "msg" is a
- * NULL pointer, return generic error message. (For "err", pass the result of
- * functions that return non-zero as an error status and thereby avoid bloating
- * up the code with if-error-conditionals.)
+/* If "err" == 0, do nothing. Else, clean up and exit with an error message that
+ * consists, first, of "msg" or (if "msg" is a NULL pointer) a generic "Details
+ * unknown", secondly of "err" as the "internal error code", and thirdly of
+ * errno if it is non-zero.