rekey on sealed/uninitialized vault fails with 'not configured for highly-available mode'

[thomasmitchell]

Describe the bug
Commit 3c1d57d reintroduced issue #2334
Namely, that attempting to rekey a sealed or uninitialized vault results in a 500 and a complaint about not being in HA mode instead of a 503.

To Reproduce
Steps to reproduce the behavior:

1. Start vault in server mode.
2. Attempt to start a rekey operation.

Expected behavior
I expected a 503 to be returned.

Environment:

• Vault Server Version (retrieve with vault status): 0.10.3
• Vault CLI Version (retrieve with vault version): 0.10.3
• Server Operating System/Architecture: darwin/amd64

Vault server configuration file(s):

backend "inmem" {}

disable_mlock = true

listener "tcp" {
  address = "127.0.0.1:8208"
  tls_cert_file = "/var/folders/k5/0b3rp3sd2xx17p2rls0cvjz40000gn/T/vaultkv-test-cert627700549"
  tls_key_file = "/var/folders/k5/0b3rp3sd2xx17p2rls0cvjz40000gn/T/vaultkv-test-key295533024"
}

Additional context
Add any other context about the problem here.