New Root URL: Don't crash when try add root resource with URL matching existing root resource

[davidfstr]

Repro Steps:

• Select an existing root resource (⚓ ) in the Entity Tree.
• Press "New Root URL...". Dialog opens with URL auto-populated to match the existing root resource.
• Press OK button.

Expected Results:

• Error dialog appears saying that "The X root URL already exists." and returns to the prior dialog.

Actual Results:

• Nothing happens in UI. Following traceback printed to terminal (if visible):

  Traceback (most recent call last):
  File "/Users/davidf/Projects/CrystalWebArchiver/Code/src/crystal/util/wx_bind.py", line 34, in bind_target
  yield
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/contextlib.py", line 79, in inner
  return func(*args, **kwds)
  File "/Users/davidf/Projects/CrystalWebArchiver/Code/src/crystal/browser/addrooturl.py", line 99, in _on_button
  self._on_ok(event)
  File "/Users/davidf/Projects/CrystalWebArchiver/Code/src/crystal/browser/addrooturl.py", line 109, in _on_ok
  self._on_finish(name, url)
  File "/Users/davidf/Projects/CrystalWebArchiver/Code/src/crystal/browser/__init__.py", line 437, in _on_add_url_dialog_ok
  RootResource(self.project, name, Resource(self.project, url))
  File "/Users/davidf/Projects/CrystalWebArchiver/Code/src/crystal/model.py", line 2129, in __new__
  raise RootResource.AlreadyExists
  crystal.model.RootResource.AlreadyExists

Priority: Medium/High

• Very easy to trigger this issue.
• Issue is quite confusing (because UI becomes unresponsive).