Wrapping errors instead of merely embedding error messages allows
calling code to use `errors.Is` and `errors.As` to more precisely check
the reasons for various errors instead of having to rely only on
substring searches.

We implement our own wrapper error to retain backward compatibility
prior to Go 1.13, where there is no support for the "%w" format string
in `fmt.Errorf`.

There's no particular reason to make error-wrapping be part of Mage's
public interface, so now it's kept in the internal package. One
consolation was needed to avoid circular imports, which is that when
`mg.F` panics, it does not _wrap_ the JSON-marshalling error.

Instead of having the caller create an error with fmt.Errorf and then
pass it to WrapError, we now have WrapErrorf that wraps the error and
formats a string error at once.