Better error output and exit code

[toshimaru]

closes #57

Before

$ nyan x go.mod
open x: no such file or directory
module github.com/toshimaru/nyan

go 1.13

require (
	github.com/alecthomas/chroma v0.6.8
	github.com/alecthomas/repr v0.0.0-20181024024818-d37bc2a10ba1 // indirect
	github.com/dlclark/regexp2 v1.1.8 // indirect
	github.com/mattn/go-colorable v0.1.4
	github.com/mattn/go-isatty v0.0.10
	github.com/spf13/cobra v0.0.5
	github.com/stretchr/testify v1.3.0
)

$ echo $?
0

$ nyan x
open x: no such file or directory
Error: open x: no such file or directory
Usage:
  nyan [OPTION]... [FILE]... [flags]

Examples:
   ...

After

$  go run main.go -- x go.mod
Error: open x: no such file or directory
module github.com/toshimaru/nyan

go 1.13

require (
	github.com/alecthomas/chroma v0.6.8
	github.com/alecthomas/repr v0.0.0-20181024024818-d37bc2a10ba1 // indirect
	github.com/dlclark/regexp2 v1.1.8 // indirect
	github.com/mattn/go-colorable v0.1.4
	github.com/mattn/go-isatty v0.0.10
	github.com/spf13/cobra v0.0.5
	github.com/stretchr/testify v1.3.0
)
exit status 1

$ echo $?
1

$ go run main.go -- x
Error: open x: no such file or directory
exit status 1