Arcpy: how to find out that a .mxd is invalid? Try: except: does not work

Question

I'm cleaning up a 100 GB network disc full of (old) ArcMap projects, some dating back to 2004 (!). I found out that there are .mxd's on that disc that ArcMap can't handle, because it crashes, throwing the error message in this picture

If I want to test such a .mxd on its validity using Arcpy, I use this simple code (that works for valid .mxd's) and run it in IDLE:

import arcpy
mxd=<name>
try:
  inp = arcpy.mapping.MapDocument(mxd)
  print "OK"
except:
  print "error"

Clearly, it won't print "OK" for an invalid .mxd, but strange enough it does not print "error" either, what you would expect. Instead, IDLE only shows this:

=============================== RESTART: Shell ===============================
>>>

Questions:

1. Why is this try: except: not working?
2. How to find out, using Arcpy, that a .mxd is invalid? So I can find all invalid .mxd's on that 100 GB disc!