[enhancement](err-msg) Clarify error msg for forward queries (#40802) (…

…#41474)

## Proposed changes

pick: #40802

fe/fe-core/src/main/java/org/apache/doris/qe/MasterOpExecutor.java

@@ -303,7 +303,9 @@ public static class ForwardToMasterException extends RuntimeException {
  .put(TTransportException.UNKNOWN, "Unknown exception")
  .put(TTransportException.NOT_OPEN, "Connection is not open")
  .put(TTransportException.ALREADY_OPEN, "Connection has already opened up")
- .put(TTransportException.TIMED_OUT, "Connection timeout")
+ .put(TTransportException.TIMED_OUT,
+ "Connection timeout, please check network state or enlarge session variable:"
+ + "`query_timeout`/`insert_timeout`")
  .put(TTransportException.END_OF_FILE, "EOF")
  .put(TTransportException.CORRUPTED_DATA, "Corrupted data")
  .build();