Enhance the help message for invalid command in datafusion-cli (apache#18603)

## Which issue does this PR close?

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- Closes apache#18375 .

## What changes are included in this PR?

Enhance the help message for invalid command, add the `help` command in
the help message.

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## Are these changes tested?

Manual test

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## Are there any user-facing changes?

No

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Author: klion26

datafusion-cli/src/exec.rs

@@ -153,7 +153,7 @@ pub async fn exec_from_repl(
                                     }
                                 } else {
                                     eprintln!(
-                                        "'\\{}' is not a valid command",
+                                        "'\\{}' is not a valid command, you can use '\\?' to see all commands",
                                         &line[1..]
                                     );
                                 }
@@ -168,7 +168,7 @@ pub async fn exec_from_repl(
                         }
                     }
                 } else {
-                    eprintln!("'\\{}' is not a valid command", &line[1..]);
+                    eprintln!("'\\{}' is not a valid command, you can use '\\?' to see all commands", &line[1..]);
                 }
             }
             Ok(line) => {