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.

CHANGELOG.md

@@ -24,12 +24,16 @@ and this project adheres to
 - Parsing subscriptions (the execution always fails yet).
 - `Error.ResponseEventStream`, `Type.Out.Resolve`, `Type.Out.Subscribe` and
   `Type.Out.SourceEventStream` define subscription resolvers.
+- `Error.ResolverException` is an exception that can be thrown by (field value
+  and event stream) resolvers to signalize an error. Other exceptions will
+  escape.
 
 ## Changed
 - `Type.Out.Resolver`: Interface fields don't have resolvers, object fields
   have value resolvers, root subscription type resolvers need an additional
   resolver that creates an event stream. `Resolver` represents these differences
-  now and pairs a field with the function(s).
+  now and pairs a field with the function(s). Resolvers don't have `ExceptT`,
+  errors are handled with `MonadThrow`/`MonadCatch`.
 - All code from `Trans` is moved to `Type.Out` and exported by `Type` and
   `Type.Out`.
 - `AST.Core` contained only `Arguments` which was moved to `Type.Definition`.
@@ -43,6 +47,8 @@ and this project adheres to
   variable names is changed back to JSON since it is a common format and it
   saves additional conversions. Custom format still can be used with the
   underlying functions (in the `Execute` module).
+- The constraint of the base monad was changed to `MonadCatch` (and it implies
+  `MonadThrow`).
 
 ## Removed
 - `Trans.ActionT` is an unneeded layer of complexity. `Type.Out.Resolver`