Comment on the return type of SharedStackError.getLocation().

That method should probably be renamed.
Added Stack.getNumFrames().
Updated comment on return value from ErrorParser.parse to match what is actually returned.

Also returning a tree element tuple at all return points.
Initialised gui module with __init__.py.
Implemented gui.TreePanel.
Implemented demo application of TreePanel.

Displays the error tree in a proper window.