Let ReadP carry a failure message

[Dannyu NDos]

Well, I once tried implementing a parser that evaluates integer
addition, multiplication, exponential, tetration, pentation, and so on
infinitely. The operators were + with precedence 6, * with precedence
7, ^ with precedence 8, ^^ with precedence 9 (for tetration), ^^^ with
precedence 10 (for pentation), ^^^^ with precedence 11 (for hexation),
and so on infinitely. I've not succeeded implementing it using
ordinary ReadP.