Don't rescue IPAddr::InvalidAddressError

`IPAddr::InvalidAddressError` does not exist in Ruby 1.9.3 and fails for JRuby in 1.9 mode.
    
As `IPAddr::InvalidAddressError` is a subclass of `ArgumentError` (via `IPAddr::Error`) just rescuing `ArgumentError` is fine.
I think we can leave out the last paragraph. If someone is interested in the solution he can look at the commit.