Improve error for label_category duplicate

[pegerto]

Hi there,

When a label category is duplicated the message needs to be propagated correctly,

To reproduce one can use the following terraform code

resource skytap_label_category "duplabel1" {
  name = "tftest-dup"
  single_value = true
}

resource skytap_label_category "duplabel2" {
  name = "tftest-dup"
  single_value = true
}