cli: don't panic when handling error

Close #2886

Signed-off-by: Ekaterina Pavlova <ekt@morphbits.io>

cli/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"fmt"
 	"os"
 
 	"github.com/nspcc-dev/neo-go/cli/app"
@@ -10,6 +11,7 @@ func main() {
 	ctl := app.New()
 
 	if err := ctl.Run(os.Args); err != nil {
-		panic(err)
+		fmt.Fprintln(ctl.ErrWriter, err)
+		os.Exit(1)
 	}
 }