[#943] Add tutorial section on error handling in extensions
[#943] Add __init__.py files to examples/iauthfunctions

These are needed for the extension to load successfully.
[#943] Move the example iauthfunctions extension

Put it in the same place as example_idatasetform etc.
[#943] Rename example_iauthfunctions plugin modules

Adding a v makes it clearer that they're successive versions of the same
module.
[#943] Update includes in writing-extensions docs
[#943] Delete stray __init__.py file
[#943] Edit the writing extensions tutorial

Lots of minor edits
[#943] Show .. todo:: items in Sphinx output