Improve error message when variant constructor is not prepended with pipe

[zth]

This is a syntax error because the constructor needs a preceding | when there's an attribute:

type variant = @as("one") One

But the error message is very cryptic:

[E] Line 1, column 26:
Did you mean `one` instead of `One`?

We should improve this error to say what you need to do.

https://rescript-lang.org/try?version=v12.0.0-alpha.14&module=esmodule&code=C4TwDgpgBAbghgJwJZwHbCgXigATgZwAoAiAe1QmIEooB5CoA