Improved IAB error

app/src/fdroid/java/eu/faircode/email/ActivityBilling.java

@@ -540,6 +540,9 @@ public class ActivityBilling extends ActivityBase implements /*PurchasesUpdatedL
         else {
             message = getBillingResponseText(result);
 
+            if (result.getResponseCode() == BillingClient.BillingResponseCode.BILLING_UNAVAILABLE)
+                message += " Is the Play Store app logged into the account used to install the app?";
+
             String debug = result.getDebugMessage();
             if (!TextUtils.isEmpty(debug))
                 message += " " + debug;

app/src/play/java/eu/faircode/email/ActivityBilling.java

@@ -539,6 +539,9 @@ public class ActivityBilling extends ActivityBase implements PurchasesUpdatedLis
         else {
             message = getBillingResponseText(result);
 
+            if (result.getResponseCode() == BillingClient.BillingResponseCode.BILLING_UNAVAILABLE)
+                message += " Is the Play Store app logged into the account used to install the app?";
+
             String debug = result.getDebugMessage();
             if (!TextUtils.isEmpty(debug))
                 message += " " + debug;