Improve error when using '@deriving(accessors)` on a variant with a record arg

[JonoPrest]

Closes #6704

Previously

@deriving(accessors)
type t = Struct({a: int})

would error with:

Fatal error: exception File "jscomp/frontend/ast_derive_projector.ml", line 57, characters 42-48: Assertion failed

now we get:

  We've found a bug for you!
  /.../fixtures/DerivingAccessorsRecordParam.res:2:10-25
                                                                                                               
  1 │ @deriving(accessors)
  2 │ type t = Struct({a: int})
  3 │
                                                                                                               
  @deriving(accessors) from a variant record argument is unsupported. Either define the record type separatelyfrom the variant type or use a positional argument.

[fhammerschmidt]

it actually closes #6704 ;-)
very nice work!

[JonoPrest]

it actually closes #6704 ;-) very nice work!

Thanks! Sorry I mis-typed that!

[zth]

Very nice! 🎉

[cristianoc]

Why does this appear twice?
Are there 2 different ways to trigger the error?
In any case better refactor the error message printing and avoid duplication.

[JonoPrest]

Ah yes, sorry I just copy pasted 👍🏼, the one is for structure gen the other is for signature gen. If it hits the case in one it will also in the other so you will only see one message.

[JonoPrest]

@cristianoc I've refactored it. See the following commit. Thanks!