In GF.Compile.CheckGrammar, use a new topological sorting function that
groups independent judgements, allowing them all to be checked before
continuing or reporting errors.
Reimplemented it with the new function accumulateError.
Also keeping the formatting of errors and warnings unchanged for now, to avoid
potentially causing problems in the GF Eclipse Plugin.
In addition to warnings, the Check monad in GF.Infra.CheckM can now accumulate
errors. There are two new functions
checkAccumError: Message -> Check ()
accumulateError :: (a -> Check a) -> a -> Check a
The former (with the same type as checkWarn) is used to report an accumulated
(nonfatal) error. The latter converts fatal errors into accumulated errors.
Accumulated errors are reported as regular errors by runCheck.
Also, the Check monad type has been made abstract.
