Made the error management for loading the JSON config file work as one might
expect it should. Expanded the error reporting expectations to include specific
file reading and JSON decoding errors.
The difficulties that will be had in handling the multiple either values will
motivate latter sections RE ExceptT (EitherT) transformers. However there is
nothing here that should be beyond people that do not know how transformers
work.
Added more doctests to the different moving parts to make things a bit clearer.
Added doctests to the cabal files and included a 'doctests.hs' to the tests
section.
default.nix files have been updated with the bonus dependency.
Added level05: installing a DB layer.
Updated the cabal files with a nice ghc-option of '-Wall -Werror'.
Added some more comments to the level04 testing page.
Level 04 contains some testing fun.
Removed spurious yaml file from level03.
Update README.
Have noticed irritating snafu RE the app structure. Not built as a
library in other levels, will fix.
