Return proper exit code for exec errors

Exec erros from the exec() syscall in the container's init should be
treated as if the container ran but couldn't execute the process for the
user instead of returning a libcontainer error as if it was an issue in
the library.

Before specifying different commands like `/etc`, `asldfkjasdlfj`, or
`/alsdjfkasdlfj` would always return 1 on the command line with a
libcontainer specific error message.  Now they return the correct
message and exit status defined for unix processes.

Example:

```bash
root@deathstar:/containers/redis# runc start test
exec: "/asdlfkjasldkfj": file does not exist
root@deathstar:/containers/redis# echo $?
127
root@deathstar:/containers/redis# runc start test
exec: "asdlfkjasldkfj": executable file not found in $PATH
root@deathstar:/containers/redis# echo $?
127
root@deathstar:/containers/redis# runc start test
exec: "/etc": permission denied
root@deathstar:/containers/redis# echo $?
126
```

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

utils.go

@@ -6,7 +6,9 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"os/exec"
 	"path/filepath"
+	"strconv"
 	"syscall"
 
 	"github.com/Sirupsen/logrus"
@@ -161,6 +163,22 @@ func getContainer(context *cli.Context) (libcontainer.Container, error) {
 // fatal prints the error's details if it is a libcontainer specific error type
 // then exits the program with an exit status of 1.
 func fatal(err error) {
+	// return proper unix error codes
+	if exerr, ok := err.(*exec.Error); ok {
+		switch exerr.Err {
+		case os.ErrPermission:
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(126)
+		case exec.ErrNotFound:
+			fmt.Fprintln(os.Stderr, err)
+			os.Exit(127)
+		default:
+			if os.IsNotExist(exerr.Err) {
+				fmt.Fprintf(os.Stderr, "exec: %s: %v\n", strconv.Quote(exerr.Name), os.ErrNotExist)
+				os.Exit(127)
+			}
+		}
+	}
 	if lerr, ok := err.(libcontainer.Error); ok {
 		lerr.Detail(os.Stderr)
 		os.Exit(1)