Improve exception message when DruidLeaderClient doesn't find leader node (#14775)

The existing exception message No known server thrown in DruidLeaderClient is unhelpful.

server/src/main/java/org/apache/druid/discovery/DruidLeaderClient.java

@@ -264,7 +264,11 @@ public class DruidLeaderClient
     );
 
     if (leader == null) {
-      throw new IOE("No known server");
+      throw new IOE(
+          "A leader node could not be found for [%s] service. "
+          + "Check logs of service [%s] to confirm it is healthy.",
+          nodeRoleToWatch, nodeRoleToWatch
+      );
     } else {
       return leader;
     }

server/src/test/java/org/apache/druid/discovery/DruidLeaderClientTest.java

@@ -159,7 +159,9 @@ public class DruidLeaderClientTest extends BaseJettyTest
     druidLeaderClient.start();
 
     expectedException.expect(IOException.class);
-    expectedException.expectMessage("No known server");
+    expectedException.expectMessage(
+        "A leader node could not be found for [PEON] service. "
+        + "Check logs of service [PEON] to confirm it is healthy.");
     druidLeaderClient.makeRequest(HttpMethod.POST, "/simple/direct");
   }