Print what is the validation that failed in the error message

[3v0k4]

First of all, thanks for your work! I really appreciated the simplicity and power of this gem.

Describe the problem

Currently, the following validation:

argument :foo do
  validate /\d+/
end

Would return the following error when passed --foo bar:

"value of bar fails validation for '--foo' option"

But I have to check the source to know what is expected of --foo. Or hope it was documented in the desc.

How would the new feature work?

I'd like to either get something like (extracted with some Ruby magic):

"value of bar fails validation for '--foo' option, expected /\d+/"

Or, have the opportunity to specify the error message myself with something like:

argument :foo do
  validate /\d+/, "expected digit"
end

I'm happy to come up with a better API (and potentially a PR) for either version if this is a change that you'd like to have.

Drawbacks

Nothing as of now. But we may want to consider adding the same to the help.