Token Attenuator should show explicit message if token already sealed

[malcolmstill]

If I pass a sealed token to https://www.biscuitsec.org/docs/tooling/token-attenuator/ and attempt to add a block you get an error message that says An error has happened:

It would be great if the error message stated that the token was sealed and so can't be attenuate.

Example token (test020_sealed.bc from testsuite):

EqcBCj0KBWZpbGUxCgVmaWxlMhgDIg0KCwgEEgMYgAgSAhgAIg0KCwgEEgMYgQgSAhgAIg0KCwgEEgMYgAgSAhgBEiQIABIgEFXHULGhUFk3rxU3xia6MmOZXDOmR1iqr7EnWwMS4oQaQHWVoRKh61uBpuOYhS5hGLf1uMu_9FJ3jmVRAOX7T6qNOir1L-LE-VJIeWBWdfriatvEeD4Mr8Q1IvqCOF85bAMalQEKKwoBMBgDMiQKIgoCCBsSBwgCEgMIgggSBggDEgIYABILCAQSAwiCCBICGAASJAgAEiDUQk0eEpEzoUeX40zcX6nap0wrG2eq_k-10bM-vdpa0RpARfTBT52ej6BE1ovnouyM3bg19XXHuRPsWb1jbHCsrpqQ25BkugswhCkO0MQiu7cXAJKohPXgICsx6SNbvMFlDSJCEkCfIpa5yKK-ahepkwhc6RvDyntWIW_VMsGtUkqV47nLtsU8Y7kjVdFJBJGfc-ijQ23RAEcD5tGR54nVtqdgbpYE