Improve error message when trying to use unsafe self

[dovreshef]

The error does not make it clear.

struct S;

impl S {
    fn foo(*mut self) { }
}

error: unexpected token: `*`
     fn foo(*mut self) {
            ^

[bblum]

Probably by design, but should at least give a better error message.

[brson]

Updated title to make this about improving the error message.