if launcher fails to connect silently, report an error

core/src/main/java/hudson/slaves/SlaveComputer.java

@@ -192,7 +192,6 @@ public class SlaveComputer extends Computer {
                     log.rewind();
                     try {
                         launcher.launch(SlaveComputer.this, taskListener);
-                        return null;
                     } catch (AbortException e) {
                         taskListener.error(e.getMessage());
                         throw e;
@@ -208,6 +207,10 @@ public class SlaveComputer extends Computer {
                     if (channel==null)
                         offlineCause = new OfflineCause.LaunchFailed();
                 }
+
+                if (channel==null)
+                    throw new IOException("Slave failed to connect, even though the launcher didn't report it. See the log output for details.");
+                return null;
             }
         });
     }