chore(spanner): add extra logging in executor (googleapis#3205)

Author: harshachinta

google-cloud-spanner-executor/src/main/java/com/google/cloud/executor/spanner/CloudClientExecutor.java

@@ -2665,6 +2665,7 @@ private Status processResults(
  executionContext.finishRead(Status.OK);
  return sender.finishWithOK();
  } catch (SpannerException e) {
+ LOGGER.log(Level.WARNING, "Encountered exception: ", e);
  Status status = toStatus(e);
  LOGGER.log(
  Level.WARNING,