fix(chromeos): Fix node-ssh exit code interpretation (#92)

Since upgrading node-ssh, we sometimes get an exit code of 0, other
times an exit code of null. This makes the tool more flexible in
interpreting those codes.

backends/chromeos/chromeos-utils.js

@@ -211,8 +211,9 @@ async function executeRemoteCommand(log, ssh, argv) {
 
   const output = await ssh.exec(executable, args, options);
 
-  // output.code == 0 means success.
-  if (output.code != 0) {
+  // output.code == 0 or output.code == null means success.
+  const success = output.code == 0 || output.code == null;
+  if (!success) {
     log.error(`Remote command ${argv.join(' ')} ` +
               `failed with exit code ${output.code} ` +
               `and full output: ${output.stdout} ${output.stderr}`);