When the Vault Client gets `invalid token`, consul template should exit

Even when consul-template has been correctly (or incorrectly) configured to retry forever, in the event of trying to read from Vault and getting an `invalid token` error, consul-template should probably exit non-zero.

There's recoverable errors, and unrecoverable errors- an invalid token error smells like it's unrecoverable to me.

See Issue 2 here: https://github.com/hashicorp/nomad/issues/23305#issuecomment-2643731565

Nomad special cases a *_bunch_* of errors to consider them fatal and to stop retrying- there is a good chance the same logic applies here as well, see here: hashicorp/nomad#24409 and hashicorp/nomad#25040.