libsyntax: better error for lifetimes in patterns

libsyntax: better error for lifetimes in patterns

Previously, if you copied a signature from a trait definition such as:

```
fn foo<'a>(&'a Bar) -> bool {}
```

and moved it into an `impl`, there would be an error message:

"unexpected token `'a`"

Adding to the error message that a pattern is expected should help
users to find the actual problem with using a lifetime here.