Show launcher output on remote agent exec failure

Remote agent launch may fail for several reasons. Most common one is the
agent not being installed on remote, but it may also come from a
launcher failure. Show launcher output to help debugging.

Author: mbriand

src/main/java/com/cloudbees/jenkins/plugins/sshagent/exec/ExecRemoteAgent.java

@@ -63,7 +63,8 @@ public class ExecRemoteAgent implements RemoteAgent {
         ByteArrayOutputStream baos = new ByteArrayOutputStream();
         if (launcherProvider.getLauncher().launch().cmds("ssh-agent").stdout(baos).start()
                 .joinWithTimeout(1, TimeUnit.MINUTES, listener) != 0) {
-            throw new AbortException("Failed to run ssh-agent");
+            String reason = new String(baos.toByteArray(), StandardCharsets.US_ASCII);
+            throw new AbortException("Failed to run ssh-agent: " + reason);
         }
         agentEnv = parseAgentEnv(new String(baos.toByteArray(), StandardCharsets.US_ASCII), listener); // TODO could include local filenames, better to look up remote charset