fix: catch ExecutionException for op.getName (#1729)

Following exception is thrown: java.util.concurrent.ExecutionException: com.google.api.gax.rpc.FailedPreconditionException: io.grpc.StatusRuntimeException: FAILED_PRECONDITION. So, changing the catch to catch ExecutionException as well. Catch both SpannerException and ExecutionException. Co-authored-by: Knut Olav Løite <koloite@gmail.com>

Author: rajatbhatta

google-cloud-spanner/src/test/java/com/google/cloud/spanner/it/slow/ITBackupTest.java

@@ -467,7 +467,7 @@ public void test02_RetryNonIdempotentRpcsReturningLongRunningOperations() throws
  }
  try {
  client.cancelOperation(op.getName());
- } catch (SpannerException e) {
+ } catch (SpannerException | ExecutionException e) {
  // Ignore, this can happen, as the restore operation sometimes fails to start if there
  // is already a restore operation running on the instance.
  }