Improving --use-clipboard to provide error message and exit

src/Application.vala

@@ -68,7 +68,10 @@ public class Annotator : Gtk.Application {
         Process.exit( 1 );
       }
     } else if( use_clipboard ) {
-      appwin.do_paste();
+      if( !appwin.do_paste_image() ) {
+        stderr.printf( _( "\nERROR:  Image does not exist on the clipboard\n" ) );
+        Process.exit( 1 );
+      }
     } else if( take_screenshot ) {
       appwin.do_screenshot();
     }

src/Clipboard.vala

@@ -72,9 +72,10 @@ public class AnnotatorClipboard {
   }
 
   /* Called to paste current item in clipboard to the given Canvas */
-  public static void paste( Editor editor ) {
+  public static bool paste( Editor editor, bool image_only = false ) {
 
     var clipboard = Display.get_default().get_clipboard();
+    var retval    = false;
 
     try {
       if( clipboard.get_formats().contain_mime_type( "image/png" ) ) {
@@ -85,21 +86,26 @@ public class AnnotatorClipboard {
             editor.paste_image( pixbuf, true );
           }
         });
-      } else if( clipboard.get_formats().contain_mime_type( "application/xml" ) ) {
+        retval = true;
+      } else if( !image_only && clipboard.get_formats().contain_mime_type( "application/xml" ) ) {
         clipboard.read_async.begin( {"application/xml"}, 0, null, (obj, res) => {
           string str;
           var stream   = clipboard.read_async.end( res, out str );
           var contents = Utils.read_stream( stream );
           editor.paste_items( contents );
         });
-      } else if( clipboard.get_formats().contain_gtype( Type.STRING ) ) {
+        retval = true;
+      } else if( !image_only && clipboard.get_formats().contain_gtype( Type.STRING ) ) {
         clipboard.read_text_async.begin( null, (obj, res) => {
           var text = clipboard.read_text_async.end( res );
           editor.paste_text( text );
         });
+        retval = true;
       }
     } catch( Error e ) {}
 
+    return( retval );
+
   }
 
 }

src/MainWindow.vala

@@ -505,12 +505,25 @@ public class MainWindow : Gtk.ApplicationWindow {
     _editor.canvas.do_cut();
   }
 
-  /* Pastes text or images to the editor */
+  /* Pastes clipboard contents to the editor.  This may also paste only images, if specified. */
+  private bool do_paste_internal( bool image_only ) {
+    if( AnnotatorClipboard.paste( _editor, image_only ) ) {
+      _welcome.sensitive = false;
+      _zoom_btn.set_sensitive( true );
+      _export_btn.set_sensitive( true );
+      return( true );
+    }
+    return( false );
+  }
+
+  /* Pastes text, images or items to the editor */
   public void do_paste() {
-    _welcome.sensitive = false;
-    AnnotatorClipboard.paste( _editor );
-    _zoom_btn.set_sensitive( true );
-    _export_btn.set_sensitive( true );
+    do_paste_internal( false );
+  }
+
+  /* Pasts only an image from the clipboard to the editor */
+  public bool do_paste_image() {
+    return( do_paste_internal( true ) );
   }
 
   public void do_screenshot() {