[BUGFIX] cli: Improve error message for 'k3d get-kubeconfig'

[andyz-dev]

Before this change, command:
$ k3d create -name test
$ k3d get-kubeconfig

Produces the following error message:

panic: runtime error: index out of range

goroutine 1 [running]:
github.com/rancher/k3d/cli.GetKubeConfig(0xc00031a160, 0x0, 0x0)
	/Users/azhou/projs/k3d/cli/commands.go:335 +0x105d
github.com/urfave/cli.HandleAction(0x13e7ca0, 0x148f0b0, 0xc00031a160, 0xc000304000, 0x0)
	/Users/azhou/projs/rcloud/pkg/mod/github.com/urfave/cli@v1.20.0/app.go:490 +0xc8
github.com/urfave/cli.Command.Run(0x1477cb5, 0xe, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1480cd8, 0x23, 0x0, ...)
	/Users/azhou/projs/rcloud/pkg/mod/github.com/urfave/cli@v1.20.0/command.go:210 +0x996
github.com/urfave/cli.(*App).Run(0xc0000bc1a0, 0xc00000e060, 0x2, 0x2, 0x0, 0x0)
	/Users/azhou/projs/rcloud/pkg/mod/github.com/urfave/cli@v1.20.0/app.go:255 +0x6af
main.main()
	/Users/azhou/projs/k3d/main.go:185 +0x11e0

This patch improve the error message by avoid the panic. Now the output
becomes:

019/05/02 12:05:30 No server container for cluster k3s_default

[cloudnautique]

Same fix as #21

[andyz-dev]

Same fix as #21

Well, it is the same bug, but slightly different fix. IMHO, #22 produces cleaner error message.

For the same test cases mentioned in the commit message, #21 would have produced:

2019/05/02 15:16:26 Couldn't get server container for cluster: k3s_default

Notice the extra added to the output.

In case k3d maintainer choose to merge this pull request, I'd like to request that we acknowledge the effort of #21 that identified the same underlining issue, and roll it into the commit message.

[iwilltry42]

Thank you for this :) Seems like I missed that one