vault status panics for HA server when /sys/leader returns 500

[jgiles]

Environment:

• Vault Version: Vault v0.9.3 ('5acd6a21d5a69ab49d0f7c0bf540123a9b2c696d')
• Operating System/Architecture: OSX

Expected Behavior:
Display error from server rather than panicking.

Actual Behavior:
Panics.

Steps to Reproduce:

1. Get your HA cluster into a state where leader election is busted (e.g. my server logs are spamming [ERROR] core: failed to acquire lock: error=failed to create session: Unexpected response code: 500 (No known Consul servers))

2. Try to check the Vault status.

$ curl --cacert $VAULT_CACERT $VAULT_ADDR/v1/sys/leader
{"errors":["Unexpected response code: 500"]}

$ vault status
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x25c6624]

goroutine 1 [running]:
github.com/hashicorp/vault/command.OutputSealStatus(0x3d1e2a0, 0xc420097e50, 0xc4201c36c0, 0xc4205dc980, 0x0)
        /private/tmp/vault-20180128-5063-cdu8kz/src/github.com/hashicorp/vault/command/format.go:244 +0x6d4
github.com/hashicorp/vault/command.(*StatusCommand).Run(0xc4201a93e0, 0xc420098010, 0x0, 0x0, 0xc42063e2a0)
        /private/tmp/vault-20180128-5063-cdu8kz/src/github.com/hashicorp/vault/command/status.go:82 +0x1e2
github.com/hashicorp/vault/vendor/github.com/mitchellh/cli.(*CLI).Run(0xc42042db80, 0xc42042db80, 0xc420640c40, 0xc420180780)
        /private/tmp/vault-20180128-5063-cdu8kz/src/github.com/hashicorp/vault/vendor/github.com/mitchellh/cli/cli.go:255 +0x1eb
github.com/hashicorp/vault/command.Run(0xc420098010, 0x1, 0x1, 0x2622fb9)
        /private/tmp/vault-20180128-5063-cdu8kz/src/github.com/hashicorp/vault/command/main.go:47 +0x411
main.main()
        /private/tmp/vault-20180128-5063-cdu8kz/src/github.com/hashicorp/vault/main.go:10 +0x63

Important Factoids:

I'm pretty sure the HA setup problem is user error. This issue is just about the client panic on vault status