Add post_process_errors option

[zachdaniel]

Often times, users want to conditionally decide to show errors, or modify them in some way. We have transform_errors, which gives you the Ash error struct, like %Ash.Error.Changes.InvalidChanges{}, but that can be annoying because you have to handle the conversion to to the triple, i.e {:field, "message", vars: []} before doing any work. So we should add an option that takes a callback like so:

# (can path be gotten from the form instead?)
post_process_errors: fn form, path_to_form, {field, message, vars} -> 
  # you could for example filter certain errors out
  # maybe using Phoenix.used_input?(form.raw_params, field)
  # or other custom criteria
  # or remap the field name from one thing to another
end