Improve error message for missing service name on env list

[kalbir]

When running fargate service env list -- i.e. missing the name of the service that I wanted to list the env variables for -- I got the following error:

$ fargate service env list
panic: runtime error: index out of range

goroutine 1 [running]:
github.com/jpignata/fargate/cmd.glob..func16(0x1f424c0, 0x1f68320, 0x0, 0x0)
	/Users/jp/workspace/go/src/github.com/jpignata/fargate/cmd/service_env_list.go:18 +0x6c
github.com/spf13/cobra.(*Command).execute(0x1f424c0, 0x1f68320, 0x0, 0x0, 0x1f424c0, 0x1f68320)
	/Users/jp/workspace/go/src/github.com/spf13/cobra/command.go:750 +0x2c1
github.com/spf13/cobra.(*Command).ExecuteC(0x1f41740, 0xc42001e0b8, 0x0, 0xc420014f70)
	/Users/jp/workspace/go/src/github.com/spf13/cobra/command.go:831 +0x30e
github.com/spf13/cobra.(*Command).Execute(0x1f41740, 0x0, 0x0)
	/Users/jp/workspace/go/src/github.com/spf13/cobra/command.go:784 +0x2b
github.com/jpignata/fargate/cmd.Execute()
	/Users/jp/workspace/go/src/github.com/jpignata/fargate/cmd/root.go:149 +0x5e
main.main()
	/Users/jp/workspace/go/src/github.com/jpignata/fargate/main.go:8 +0x20

In comparison, running fargate service info (again missing service name), gives this much more helpful error message:

Error: accepts 1 arg(s), received 0
Usage:
  fargate service info <service-name> [flags]

Flags:
  -h, --help   help for info

Global Flags:
      --cluster string   ECS cluster name (default "fargate")
      --no-color         Disable color output
      --region string    AWS region (default "us-east-1")
  -v, --verbose          Verbose output

Would be good if the first was the same as the second.