Provide better error message when field attribute is incorrect

[Kriskras99]

I got this error message:

error: custom attribute panicked
  --> ubiart_toolkit/src/cooked/isg/mod.rs:18:1
   |
18 | #[superstruct(variants(V22, V21, V20, V20C, V19, V18, V17, V16))]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: message: called `Result::unwrap()` on an `Err` value: Error { kind: UnknownField(ErrorUnknownField { name: "V19", did_you_mean: None }), locations: [], span: Some(#0 bytes(619472..619475)) }

which was because I had a field attribute [superstruct(V19)] instead of [superstruct(only(V19))]. It would be nice of the error message detected that the field name is also an attribute and suggests only and any other relevant attributes.
If it could also give the line number that would be even better.

[michaelsproul]

Sorry yeah the error messages in superstruct have not received any polish, so there are a lot of cases like this where we will just crash.

If I find some time I'll try threading the errors and spans around to see if that improves things. It should!