T.P.Error: define displayException for PandocError.

This is a behavior change, not an API change, since there
was already a definition that defaulted to using Show.
The change here is that we use renderError for a more human-readable
version.

Author: jgm

src/Text/Pandoc/Error.hs

@@ -68,6 +68,8 @@ data PandocError = PandocIOError Text IOError
                  deriving (Show, Typeable, Generic)
 
 instance Exception PandocError
+ where
+   displayException = T.unpack . renderError
 
 renderError :: PandocError -> Text
 renderError e =