title: Check if a file exists in scene file, if yes invoke error else force open the file
body: <p>I have a code that performs the following:</p>
  
  <ol>
  <li>opens a rigfile,  target rig file</li>
  <li>creates a Hik node and hooks the mocap joints of the rig file to the character def</li>
  <li>imports a mocap file (source rig) and creates another hik node for
  the source joints to map the character def</li>
  <li>mocap data transferred from source to target</li>
  <li>target joints are baked and source rig data is deleted.</li>
  </ol>
  
  <p>I wish to keep my  target rig file in the scene and continue procs 3-5. 
  I need to query if the target rig file exits, if yes then skip procs 1 and 2 and continue 3-5, else start over from 1 to 5.</p>
  
  <p>How do i check if the rig file already exists in the scene?
  P.s: I have just started off with coding in python in maya :)</p>