Check that app is installed before uninstalling it

When trying to uninstall an app that is not installed adb prints just
"Failure". This causes output parsing to fail and results in "panic:
runtime error: index out of range" in adb.getFailureCode().

Check whether an app is installed first and print an appropriate message
if it's not.

cmd/fdroidcl/uninstall.go

@@ -4,6 +4,7 @@
 package main
 
 import (
+	"errors"
 	"fmt"
 	"log"
 )
@@ -22,9 +23,16 @@ func runUninstall(args []string) {
 		log.Fatalf("No package names given")
 	}
 	device := mustOneDevice()
+	inst := mustInstalled(device)
 	for _, id := range args {
+		var err error
 		fmt.Printf("Uninstalling %s... ", id)
-		if err := device.Uninstall(id); err != nil {
+		if _, installed := inst[id]; installed {
+			err = device.Uninstall(id)
+		} else {
+			err = errors.New("not installed")
+		}
+		if err != nil {
 			fmt.Println()
 			log.Fatalf("Could not uninstall %s: %v", id, err)
 		}