Silence errors from file watchers when adding files

The file watcher can throw errors on Linux that are very cryptic and
actually not very useful to users, so let's hide them since there isn't
really any problem that comes from this.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/15717

pcbnew/pcb_base_frame.cpp

@@ -1172,7 +1172,11 @@ void PCB_BASE_FRAME::setFPWatcher( FOOTPRINT* aFootprint )
 
     wxLogTrace( "KICAD_LIB_WATCH", "Add watch: %s", fn.GetPath() );
 
-    m_watcher->Add( fn );
+    {
+        // Silence OS errors that come from the watcher
+        wxLogNull silence;
+        m_watcher->Add( fn );
+    }
 }