Custom derives and procedural attribute macros don't show errors

[jonas-schievink]

When we expand custom #[derive]s and attribute macros, we seem to ignore their errors.

Interestingly, if you do this:

serde::Deserialize!(struct S {
    #[serde(asd)]
    fld: u8,
});

...then the error shows up only if the invocation is inside a function body, not when it's in item position. (of course, the fact that you can call a custom derive like that is itself a bug)

[jonas-schievink]

Fixed by #10339