Improve error message when a platform doesn't support a widget.

[freakboy3742]

When a platform doesn't support a widget, the user currently gets an error of the form:

Traceback (most recent call last):
...
  File ".../toga/widgets/datepicker.py", line 43, in __init__
    self._impl = self.factory.DatePicker(interface=self)
AttributeError: module 'toga_gtk.factory' has no attribute 'DatePicker'

This has been the source of regular confusion in support channels, as this isn't a clear indication that a widget isn't supported.

This PR adds a PEP562 handler for the platform factories so that a missing widget in a factory is now reported as:

Traceback (most recent call last):
...
  File ".../toga_gtk/factory.py", line 83, in __getattr__
    raise NotImplementedError(f"Toga's GTK backend doesn't implement {name}")
NotImplementedError: Toga's GTK backend doesn't implement DatePicker

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct