showing in UI when grab was not possible

data/key-mapper.glade

@@ -602,7 +602,7 @@
                   <object class="GtkLabel">
                     <property name="visible">True</property>
                     <property name="can_focus">False</property>
-                    <property name="tooltip_text" translatable="yes">Click on a cell below and hit a key on your device.</property>
+                    <property name="tooltip_text" translatable="yes">Click on a cell below and hit a key on your device. If you have your preset active, the reported keycodes may be wrong.</property>
                     <property name="margin_top">5</property>
                     <property name="margin_bottom">5</property>
                     <property name="label" translatable="yes">Key</property>

keymapper/gtk/window.py

@@ -254,7 +254,14 @@ class Window:
         )
         if self.keycode_reader is not None:
             self.keycode_reader.stop_injecting()
-        self.keycode_reader = KeycodeInjector(self.selected_device)
+        try:
+            self.keycode_reader = KeycodeInjector(self.selected_device)
+        except OSError:
+            self.get('status_bar').push(
+                CTX_ERROR,
+                f'Could not grab device "{self.selected_device}"'
+            )
+
 
     def on_select_device(self, dropdown):
         """List all presets, create one if none exist yet."""

keymapper/injector.py

@@ -190,8 +190,12 @@ class KeycodeInjector:
             worker.start()
             # wait for the process to notify creation of the new injection
             # device, to keep the logs in order.
-            pipe[0].recv()
-            self.processes.append(worker)
+            status = pipe[0].recv()
+            if status != FAILED:
+                self.processes.append(worker)
+
+        if len(self.processes) == 0:
+            raise OSError('Could not grab any device')
 
     def stop_injecting(self):
         """Stop injecting keycodes."""