Make CLI functions return error instead of fatal out (#1379)

Before this PR, CLI functions fatal out when encountering error
instead of returning it. Such behavior makes it hard to reuse 
the functions nor writing unit tests. This PR aims to make functions 
return errors instead of error out.

cmd/fission-cli/main.go

@@ -17,11 +17,16 @@ limitations under the License.
 package main
 
 import (
+	"fmt"
 	"os"
 
 	fcli "github.com/fission/fission/pkg/fission-cli"
 )
 
 func main() {
-	fcli.NewCliApp().Run(os.Args)
+	err := fcli.NewCliApp().Run(os.Args)
+	if err != nil {
+		fmt.Printf("Error: %v\n", err.Error())
+		os.Exit(1)
+	}
 }