Tweak failure message for allow_value

allow_value currently gives an error message that looks like this:

    Expected errors to include "must be greater than 0" when amount is set to 0, got errors: ["amount must be greater than 0.0 (0)"]

This is confusing because the error message here is "must be greater than 0.0",
not "amount must be greater than 0.0".

With this commit the message changes to:

    Expected errors to include "must be greater than 0" when amount is set to 0, got errors: ["must be greater than 0.0 (attribute: "amount", value: 0)"]
Yeah, I'm basically making a new copy of `message` because I'm appending to `full_message` later on.