Parsers.Parser don't provide an easy way to override error messages. The common idiom for that is unsuspectingly very fragile. I suggest adding two new method, withFailureMessage and withErrorMessage, to provide that functionality. As for the problem, I'll let the comment in my proposed API describe it:

The semantics are slightly different than those obtained by doing | failure(msg), in that the message produced by this method will always replace the message produced, which is not guaranteed by that idiom.

For example, parser p below will always produce the designated failure message, while q will not produce it if sign is parsed but number is not.

def p = sign.? ~ number withFailureMessage  "Number expected!"
def q = sign.? ~ number | failure("Number expected!")