Error when if let arm doesn't return () could be more helpful

[tomjakubowski]

When compiling this:

fn main() {
    if let Some(x) = Some(5) {
        true
    }
}

<anon>:2:5: 4:6 error: match arms have incompatible types: expected `bool`, found `()` (expected bool, found ())
<anon>:2     if let Some(x) = Some(5) {
<anon>:3         true
<anon>:4     }
<anon>:2:5: 4:6 note: match arm with an incompatible type
<anon>:2     if let Some(x) = Some(5) {
<anon>:3         true
<anon>:4     }