Abstract error checking

[njtierney]

Resolves #529

[njtierney]

This is a pretty decently sized refactor of checking functions.

There is still more work to be done with making these simpler and easier to understand. At the moment I'm happy to merge this in and revisit this alter at another release. The goals for that revisit/spring clean should be:

• Look at various "TODO" notes to identify places for refactoring wrt checking
• Consider consolidating/categorising checking functions in checkers.R.
  • e.g., Put all installation checking functions in checkers-installation.R, put inference related checking functions in checkers-inference.R.
  • Review similar checking functions and see if they can be combined. We might not need so many custom checking functions
  • Check test coverage to ensure that the checking functions are all covered. This provides another way to look at improving test coverage
  • Revisit the error messages provided - could they provide more information?
  • Ensure all checking functions are exported in module.

I will put the above notes into a separate issue so I can remember these things!