Handling of unrecognised "file_type" configuration

[rhattersley]

Provide some sort of fail-fast feedback for invalid "file_type" values.

Currently if the configuration file contains an unsupported value for "file_type" then data.file_loader() silently returns an implicit None for the loader, resulting in a later error of:

  File ".../forest/forest/view.py", line 19, in render
    self.source.data = self.loader.image(state)
AttributeError: 'NoneType' object has no attribute 'image'

[andrewgryan]

Agreed, this is a glaring error that could be patched with a unit test checking for an Exception given a invalid file_type keyword

[github-actions]

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