This resolves most but not all warnings.  See GitHub issue 469.

In version 2.3 of the mtl library, re-export of other modules was
removed; so now those other modules need to be imported, if functions
from them are needed.  To avoid duplicate import when compiling with
earlier mtl/GHC versions, the import statements need to explicitly
list the functions imported from each module.

This abstraction was added to support building with GHC 7.10 (which
uses Control.Monad.Except) and older versions (which still used
Control.Monad.Error), without having to introduce CPP directives
everywhere.  Since we no longer support those earlier GHC versions,
there is no need for this API.