NPE in RunAndWait in 2.5.0

[tgross]

Got the following panic which didn't show up in our integration tests:

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x0 pc=0x4c90e7]

goroutine 1 [running]:
panic(0xb50160, 0xc820018080)
        /usr/local/go/src/runtime/panic.go:481 +0x3e6
github.com/joyent/containerpilot/commands.RunAndWait(0xc820180180, 0xc8201974d0, 0xc820189e20, 0x0, 0x0)
        /cp/src/github.com/joyent/containerpilot/commands/commands.go:97 +0xb87
github.com/joyent/containerpilot/core.(*App).Run(0xc82017c0e0)
        /cp/src/github.com/joyent/containerpilot/core/app.go:147 +0x37e
main.main()
        /cp/src/github.com/joyent/containerpilot/main.go:25 +0xf1

The relevant section appears to be here in RunAndWait where we're incorrectly assuming state is never nil if we get an non-nil err (which seemed reasonable but wasn't documented and turns out to be wrong). We want to capture the state if it's available but otherwise return the error without getting the state. Fix for this should be:

	state, err := c.Cmd.Process.Wait()
	if state != nil && !state.Success()) {
		if status, ok := state.Sys().(syscall.WaitStatus); ok {
			return status.ExitStatus(), err
		}
	}
	if err != nil {
		return 1, err
	}

[tgross]

I'm adding some test coverage improvements and pruning an unused branch of the error handling in the process here, but should have a PR up later today.

[tgross]

Opened #256