Adding a getLastError() method as a stderr equivalent of getLastOutput() to commandexecutor.

PHPCI/Helper/BaseCommandExecutor.php

@@ -131,6 +131,14 @@ abstract class BaseCommandExecutor implements CommandExecutor
         return implode(PHP_EOL, $this->lastOutput);
     }
 
+    /**
+     * Returns the stderr output from the last command run.
+     */
+    public function getLastError()
+    {
+        return $this->lastError;
+    }
+
     /**
      * Find a binary required by a plugin.
      * @param string $binary