SQL: handle X-Pack or X-Pack SQL not being available in a more graceful way (#34736)

Throw a different error message for a http response code of 400, but also when the error itself is of a specific type.

x-pack/plugin/sql/sql-client/src/main/java/org/elasticsearch/xpack/sql/client/JreHttpUrlConnection.java

@@ -47,6 +47,7 @@ public class JreHttpUrlConnection implements Closeable {
      * error.
      */
     public static final String SQL_STATE_BAD_SERVER = "bad_server";
+    private static final String SQL_NOT_AVAILABLE_ERROR_MESSAGE = "request [/_xpack/sql] contains unrecognized parameter: [mode]";
 
     public static <R> R http(String path, String query, ConnectionConfiguration cfg, Function<JreHttpUrlConnection, R> handler) {
         final URI uriPath = cfg.baseUri().resolve(path);  // update path if needed
@@ -176,6 +177,19 @@ public class JreHttpUrlConnection implements Closeable {
         }
         SqlExceptionType type = SqlExceptionType.fromRemoteFailureType(failure.type());
         if (type == null) {
+            // check if x-pack or sql are not available (x-pack not installed or sql not enabled)
+            // by checking the error message the server is sending back 
+            if (con.getResponseCode() >= HttpURLConnection.HTTP_BAD_REQUEST
+                    && failure.reason().contains(SQL_NOT_AVAILABLE_ERROR_MESSAGE)) {
+                return new ResponseOrException<>(new SQLException("X-Pack/SQL do not seem to be available"
+                        + " on the Elasticsearch node using the access path '"
+                        + con.getURL().getHost()
+                        + (con.getURL().getPort() > 0 ? ":" + con.getURL().getPort() : "")
+                        + "'."
+                        + " Please verify X-Pack is installed and SQL enabled. Alternatively, check if any proxy is interfering"
+                        + " the communication to Elasticsearch",
+                        SQL_STATE_BAD_SERVER));
+            }
             return new ResponseOrException<>(new SQLException("Server sent bad type ["
                     + failure.type() + "]. Original type was [" + failure.reason() + "]. ["
                     + failure.remoteTrace() + "]", SQL_STATE_BAD_SERVER));