AWS KMS vault with misconfigured user details causes panic

[michaelkernaghan]

steps to reproduce:

1. configure the signatory.yaml AWS vault with a user name that does not exist
2. start signatory

expected result:
a meaningful error message

actual result:
panic

error detail:

❯ ./signatory serve -c awskms.yaml
INFO[0000] Initializing vault                            vault=awskms vault_name=aws
panic: runtime error: index out of range [0] with length 0

goroutine 1 [running]:
github.com/ecadlabs/signatory/pkg/vault/aws.(*awsKMSIterator).Next(0xc00046fce0)
        /home/michael/signatory/pkg/vault/aws/awskms.go:108 +0x1da
github.com/ecadlabs/signatory/pkg/signatory.(*Signatory).listPublicKeys(0xc00024ea10, {0x1171d98, 0xc0000c7040})
        /home/michael/signatory/pkg/signatory/signatory.go:411 +0x310
github.com/ecadlabs/signatory/pkg/signatory.(*Signatory).ListPublicKeys(0xc00024ea10, {0x1171d98?, 0xc0000c7040?})
        /home/michael/signatory/pkg/signatory/signatory.go:447 +0x45
github.com/ecadlabs/signatory/cmd/commands.listKeys(0xc00024f9d0?, {0x116b3c0, 0xc0000ae2c0}, {0x1171d98?, 0xc0000c7040?})
        /home/michael/signatory/cmd/commands/utils.go:28 +0x33
github.com/ecadlabs/signatory/cmd/commands.NewServeCommand.func1(0xc000236280?, {0xe31ee6?, 0x2?, 0x2?})
        /home/michael/signatory/cmd/commands/serve.go:42 +0x272
github.com/spf13/cobra.(*Command).execute(0xc000236280, {0xc00039c280, 0x2, 0x2})
        /home/michael/go/pkg/mod/github.com/spf13/[email protected]/command.go:826 +0x67c
github.com/spf13/cobra.(*Command).ExecuteC(0xc000236000)
        /home/michael/go/pkg/mod/github.com/spf13/[email protected]/command.go:914 +0x2ee
github.com/spf13/cobra.(*Command).Execute(...)
        /home/michael/go/pkg/mod/github.com/spf13/[email protected]/command.go:864
main.main()
        /home/michael/signatory/cmd/signatory/main.go:49 +0x105

config

server:
  address: :6732
  utility_address: :9583

vaults:
  aws:
    driver: awskms
    config:
      user_name: Michael Kernaghan
      access_key_id: <redacted>
      secret_access_key: <redacted>
      region: us-east-1

tezos:
  tz1bQYMFieZHomNPjJvpp2g7PuhxRPDxpnFt:
    log_payloads: true
    allow:
      generic:
        - transaction
        - endorsement
        - proposals
      block:
      endorsement:
      preendorsement: