Improve error message for concurrent modifications (#17396)

Improve the error message logged when concurrent modifications
occur and --experimental_guard_against_concurrent_changes is set.

Co-authored-by: Tim Bain <tbain@google.com>
Co-authored-by: kshyanashree <109167932+kshyanashree@users.noreply.github.com>

src/main/java/com/google/devtools/build/lib/remote/RemoteSpawnCache.java

@@ -189,7 +189,9 @@ public void store(SpawnResult result) throws ExecException, InterruptedException
             try (SilentCloseable c = prof.profile("RemoteCache.checkForConcurrentModifications")) {
               checkForConcurrentModifications();
             } catch (IOException | ForbiddenActionInputException e) {
-              remoteExecutionService.report(Event.warn(e.getMessage()));
+              String msg = "Skipping uploading outputs because of concurrent modifications " +
+                  "with --experimental_guard_against_concurrent_changes enabled: " + e.getMessage();
+              remoteExecutionService.report(Event.warn(msg));
               return;
             }
           }