Implement getMessage for BulkTransferException

    Before this change, BulkTransferExceptions would be reported like this:
    ```
      Executing genrule //:foo failed: Exec failed due to IOException: null
    ```
    By implementing getMessage(), we get a more informative message, in the case
    when there is only one suppressed exception:
    ```
      Executing genrule //:foo failed: Exec failed due to IOException:
        Output download failed: Expected digest '8a39d2abd3999ab73c34db2476849cddf303ce389b35826850f9a700589b4a90/262144'
        does not match received digest 'dae4d3dce1bb0a9414f61a65ee07622fa225ec01e6efe6df6e78f9ad5c58480d/327680'.
    ```
    and in the case when there are multiple suppressed exceptions:
    ```
      Executing genrule //:foo failed: Exec failed due to IOException:
        2 errors during bulk transfer
    ```
    This helps diagnosing issues like bazelbuild/bazel#12927

    Closes #13061.

    PiperOrigin-RevId: 359250576

dataset/GitHub_Java/bazelbuild.bazel/src/main/java/com/google/devtools/build/lib/remote/BulkTransferException.java

@@ -46,4 +46,18 @@ void add(IOException e) {
   boolean onlyCausedByCacheNotFoundException() {
     return allCacheNotFoundException;
   }
+
+  static boolean isOnlyCausedByCacheNotFoundException(Exception e) {
+    return e instanceof BulkTransferException
+        && ((BulkTransferException) e).onlyCausedByCacheNotFoundException();
+  }
+
+  @Override
+  public String getMessage() {
+    // If there is only one suppressed exception, displaying that in the message should be helpful.
+    if (super.getSuppressed().length == 1) {
+      return super.getSuppressed()[0].getMessage();
+    }
+    return String.format("%d errors during bulk transfer", super.getSuppressed().length);
+  }
 }