I'm not sure I covered all the ways of throwing errors in the code.
Some functions throw ProgramErrors, some error messages.  It's still
quite a mess, but we're getting closer.  The missing cases are mostly
errors that cannot be fixed by the API client either or are a result
of wrong usage, so are in any case fatal.

One function, 'getModuleInfo', still returns a 'Maybe', but the
documentation suggests it should always succeed.  So I may change that
soon.

The spit-up of of 'checkModule' has pros and cons.  The various forms
of 'checkModule*' now become:

 checkAndLoadModule ms False ~~>
    loadModule =<< typecheckModule =<< parseModule (ms_mod_name ms)

 checkAndLoadModule ms True ~~>
   loadModule =<< desugarModule =<< typecheckModule =<< parseModule (ms_mod_name ms)

 checkModule mn False ~~>
   typecheckModule =<< parseModule mn

 checkModule mn True ~~>
   desugarModule =<< typecheckModule =<< parseModule mn

The old APIs cannot easily be provided, since the result type would be
different depending on the second argument.  However, a more
convenient API can be modelled on top of these four functions
({parse,typecheck,desugar,load}Module).