allow calling Error on the error interface

[pierrec]

This is the patch for #45.

[nalgeon]

Thanks! It treats error.Error as a special case in the compiler, which I usually try to avoid. Still, it might be worth making an exception for a fundamental type like error. I'll think about it some more and get back to you later.

[nalgeon]

Now that I think about it, the error type already gets special treatment in the compiler, since in Go it's built into the language, not just part of the standard library. So it makes sense to handle the error.Error method as well. I'm going to merge this.

[nalgeon]

Thank you!