Fix printing of optional record fields in pattern matching errors

[cknitt]

Fixes #7954

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commit: 994427f

[cristianoc]

Looks good, though an optional field of option type is very close to a time bomb.
I would be nearly tempted to issue a warning any time one uses it (that can be silenced in the 0.1% of cases where you actually need it).
The fact that the semantics is NOT what js does, makes it immediately confusing to people anyway.

[cknitt]

@cristianoc This is not only for optional fields with options.

E.g.,

type t = {b?: result<string, string>}

let f = v =>
  switch v {
  | {b: Ok(x)} => x
  | {b: Error(y)} => y
  }

currently gives the error

You forgot to handle a possible case here, for example: 
| {b: None, _}