Network error handling should be improved

[craig-rueda]

Currently, if the CLI fails to communicate with your Nelson server, you get a rather cryptic looking stack trace - would be better to have something that can be a bit more helpful.

Sample:

[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x134654c]

goroutine 1 [running]:
main.WhoAreYou(0xc42008a840, 0xc420110450, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
	/home/travis/gopath/src/github.com/Verizon/nelson-cli/src/github.com/verizon/nelson/system.go:85 +0x4dc
main.main.func19(0xc4200d8c80, 0x0, 0xc4200d8c80)
	/home/travis/gopath/src/github.com/Verizon/nelson-cli/src/github.com/verizon/nelson/main.go:755 +0x87
gopkg.in/urfave/cli%2ev1.HandleAction(0x139c880, 0xc4200c9160, 0xc4200d8c80, 0xc42006c800, 0x0)
	/home/travis/gopath/src/github.com/Verizon/nelson-cli/vendor/src/gopkg.in/urfave/cli.v1/app.go:485 +0xd4
gopkg.in/urfave/cli%2ev1.Command.Run(0x141211a, 0x7, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1420c22, 0x2b, 0x0, ...)
	/home/travis/gopath/src/github.com/Verizon/nelson-cli/vendor/src/gopkg.in/urfave/cli.v1/command.go:193 +0xb72
gopkg.in/urfave/cli%2ev1.(*App).RunAsSubcommand(0xc4200f04e0, 0xc4200d8a00, 0x0, 0x0)
	/home/travis/gopath/src/github.com/Verizon/nelson-cli/vendor/src/gopkg.in/urfave/cli.v1/app.go:374 +0xacc
gopkg.in/urfave/cli%2ev1.Command.startApp(0x1411af4, 0x6, 0x0, 0x0, 0x0, 0x0, 0x0, 0x142576b, 0x45, 0x0, ...)
	/home/travis/gopath/src/github.com/Verizon/nelson-cli/vendor/src/gopkg.in/urfave/cli.v1/command.go:280 +0x824
gopkg.in/urfave/cli%2ev1.Command.Run(0x1411af4, 0x6, 0x0, 0x0, 0x0, 0x0, 0x0, 0x142576b, 0x45, 0x0, ...)
	/home/travis/gopath/src/github.com/Verizon/nelson-cli/vendor/src/gopkg.in/urfave/cli.v1/command.go:79 +0x14c7
gopkg.in/urfave/cli%2ev1.(*App).Run(0xc4200f01a0, 0xc42000e480, 0x3, 0x3, 0x0, 0x0)
	/home/travis/gopath/src/github.com/Verizon/nelson-cli/vendor/src/gopkg.in/urfave/cli.v1/app.go:250 +0x7d0
main.main()
	/home/travis/gopath/src/github.com/Verizon/nelson-cli/src/github.com/verizon/nelson/main.go:1109 +0x458e