Show variant aliases in error message

[Mingun]

Before that PR an error message of such enum did not include aliases of possible variants:

#[derive(Deserialize)]
enum Enum {
  #[serde(alias = "C")]
  A,
  B
}

Now the error message will mention C

[Mingun]

@dtolnay, @oli-obk, can you give some feedback?

[oli-obk]

Not in the near future. The only PR I'm giving attention to right now is the serde_core one