Better error handling of errors when a world fails to open.

A common case is a corrupt (or empty) level.dat file.  This condition
wasn't properly caught, yielding a less-than-useful stack trace.  Even
more concerning is that this could happen when a user is just running
"overviewer.py" to get a world listing.

This has been fixed to improve the user experience

overviewer_core/nbt.py

@@ -189,7 +189,7 @@ class NBTFileReader(object):
             payload = self._read_tag_compound()
             
             return (name, payload)
-        except (struct.error, ValueError), e:
+        except (struct.error, ValueError, TypeError), e:
             raise CorruptNBTError("could not parse nbt: %s" % (str(e),))
 
 # For reference, the MCR format is outlined at