Improve error message on day parse fail
Improve error message on day parse fail

Before:

    *> parse utcTime "asd"
    Fail "asd" [] "Failed reading: takeWhile1"

After:

    *> parse utcTime "asd"
    Fail "asd" [] "Failed reading: date must be of form [+,-]YYYY-MM-DD"

There's probably a better don't-repeat-yourself refactor here of the error message, but my parsec-fu is not strong.

This is important in particular because this parser's error messages often get sent directly to users when serving json APIs via aeson.
dmjio/aeson,sol/aeson,sol/aeson,sol/aeson