Custom scalar parser errors

[maartenvanvliet]

See #856

The parse/1 could only return :error in case of a failure when parsing a custom scalar. This PR fixes that by also allowing an error tuple as a return value.

The error is stored in the flag instead of the errors field. This was more in line with how the current code was set up. The cases where the parse error handles in input objects and input lists are also handled.

[benwilson512]

@maartenvanvliet I'm generally 👍 to this, what's the consensus amongst the other implementations with respect to this feature?

[maartenvanvliet]

The graphql reference implementation supports custom errors, e.g.: https://github.com/graphql/graphql-js/blob/main/src/type/scalars.ts#L29

The ruby implementation as well: https://github.com/rmosolgo/graphql-ruby/blob/master/lib/graphql/types/iso_8601_date_time.rb#L49

[benwilson512]

Great, thanks!