Improve message produced by eitherResult for Partial

[chris-martin]

When a parser result is Partial, previously the eitherResult function showed a generic error message that includes no useful information other than a generic "incomplete input" message.

For example, if we let

p = (,) <$> letter <*> digit <?> "thing"

then

eitherResult (parse p "a") = Left "Result: incomplete input"

Attoparsec is already capable of generating more informative error information for this example, as we can demonstrate by using parseOnly:

parseOnly p "a" = "thing > digit: not enough input"

This change brings that same error message to eitherResult by simply feeding mempty into the continuation. With this change, eitherResult now gives the same output as parseOnly.

eitherResult (parse p "a") = "thing > digit: not enough input"

[chris-martin]

Is this change okay? Is there anything I can do to improve it?