Constrain base monad to MonadCatch

Let's try MonadThrow/MonadCatch. It looks nice at a first glance. The
monad transformer stack contains only the ReaderT, less lifts are
required. Exception subtyping is easier, the user can (and should)
define custom error types and throw them. And it is still possible to
use pure error handling, if someone doesn't like runtime exceptions or
need to run a query in a pure environment.

Fixes #42.

graphql.cabal

@@ -4,7 +4,7 @@ cabal-version: 1.12
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 38e16611476c6163a049a4ddbaef34cf3fdef8f85d25f7bcaed839372c9fdf75
+-- hash: f3469205f704a81ee0f55655758cf588a9e9eb52303dadd58def32a2eb207696
 
 name:           graphql
 version:        0.8.0.0
@@ -64,6 +64,7 @@ library
     , base >=4.7 && <5
     , conduit
     , containers
+    , exceptions
     , megaparsec
     , parser-combinators
     , scientific
@@ -100,6 +101,7 @@ test-suite tasty
     , base >=4.7 && <5
     , conduit
     , containers
+    , exceptions
     , graphql
     , hspec
     , hspec-expectations