docs: Document options for presenting validation error messages with the envelop Masked Error plugin

[dthyresson]

When implementing the ability add GraphQL Scalars to a RedwoodJS applications GraphQL schema (see: redwoodjs/redwood#4053) I found that the error message provided when a scalar fails validation is a TypeError wrapped as the original error of the GraphQL error when using the envelop Masked Error plug-in.

Need to document the options for how best to handle errors and messages when using the Masked Errors plug-in.

You can allow the error have a message like:

 Variable "$input" got invalid value "Roobles" at "input.currency_iso_4217"; Expected type "Currency". Value is not a valid currency value: Roobles

Or instead elect to use the original error message raised by the TypeError:

Value is not a valid currency value: Roobles

The former exposes field names (which is exactly what Masked Errors intends to prevent) while the latter is much more of a message that can be shared clients side in a HTML Form error message.

[ardatan]

Now we are using GraphQLError instead of TypeError so the errors should not be masked now.