[simple-scan] Display an error, when no documents found in the feeder during start scanning

[Robert Ancell <rancell src gnome org>]

commit ca77e936a30e2a439bb4023a4df69fb541eb3420
Author: Bartosz Kosiorek <gang65 poczta onet pl>
Date:   Sat Dec 7 00:57:58 2019 +0100

    Display an error, when no documents found in the feeder during start scanning

 src/scanner.vala | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/src/scanner.vala b/src/scanner.vala
index 66a20c0..d70c5dc 100644
--- a/src/scanner.vala
+++ b/src/scanner.vala
@@ -1253,7 +1253,13 @@ public class Scanner : Object
         if (status == Sane.Status.GOOD)
             state = ScanState.GET_PARAMETERS;
         else if (status == Sane.Status.NO_DOCS)
+        {
             do_complete_document ();
+            if (page_number == 0)
+                fail_scan (status,
+                    /* Error displayed when no documents at the start of scanning */
+                    _("Document feeder empty"));
+        }
         else
         {
             warning ("Unable to start device: %s", Sane.strstatus (status));