Currently there is no way for Servant users to customize formatting of
error messages that arise when combinators can't parse URL or request
body, apart from reimplementing those combinators for themselves or
using middlewares.
This commit adds a possibility to specify custom error formatters
through Context.
Fixes#685
This commit adds an explanation and a link to the Servant Cookbook
to `genericServerT`.
Moreover, the `genericServer` and `genericServe`'s haddocks are
slightly edited to add a missing 'a'.
