Add comment to remind future self of adding new exception types to sw…

…itch-case

external-sop/src/main/java/sop/external/ExternalSOP.java

@@ -276,6 +276,7 @@ private static void mapExitCodeOrException(@Nonnull Process process) throws Inte
                         exitCode + "):\n" + errorMessage);
 
             default:
+                // Did you forget to add a case for a new exception type?
                 throw new RuntimeException("External SOP backend reported unknown exit code (" +
                         exitCode + "):\n" + errorMessage);
         }