[SPARK-29556][CORE] Avoid putting request path in error response in ErrorServlet

### What changes were proposed in this pull request?

Don't include `$path` from user query in the error response.

### Why are the changes needed?

The path could contain input that is then rendered as HTML in the error response. It's not clear whether it's exploitable, but better safe than sorry as the path info really isn't that important in this context.

### Does this PR introduce any user-facing change?

No

### How was this patch tested?

Existing tests.

Closes #26211 from srowen/SPARK-29556.

Authored-by: Sean Owen <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: srowen

core/src/main/scala/org/apache/spark/deploy/rest/RestSubmissionServer.scala

@@ -317,8 +317,7 @@ private class ErrorServlet extends RestServlet {
           versionMismatch = true
           s"Unknown protocol version '$unknownVersion'."
         case _ =>
-          // never reached
-          s"Malformed path $path."
+          "Malformed path."
       }
     msg += s" Please submit requests through http://[host]:[port]/$serverVersion/submissions/..."
     val error = handleError(msg)