Log WiFi RequestScan failures with D-Bus error detail

The RequestScan result and the underlying sd_bus error were silently
dropped, so a denied or failed scan looked identical to an empty scan
list on the WiFi setup page. Surface both so the deployed-machine
diagnosis (e.g. user not in the networkmanager group) is visible in
~/.local/radiantwave/log.txt.

src/network/network_manager.cpp

@@ -379,7 +379,11 @@ bool NetworkManager::requestScan() {
     }
     
     log("Requesting WiFi scan...");
-    return impl_->device->requestScan();
+    const bool ok = impl_->device->requestScan();
+    if (!ok) {
+        log("RequestScan failed; the network list may stay empty until it succeeds");
+    }
+    return ok;
 }
 
 std::vector<NetworkInfo> NetworkManager::getScanResults() const {

src/network/nm_device.cpp

@@ -1,5 +1,6 @@
 #include "nm_device.h"
 #include "nm_connection.h"
+#include "logger/logger.h"
 #include <iostream>
 #include <stdexcept>
 
@@ -188,6 +189,12 @@ bool WifiDevice::requestScan() {
         0  // Empty options dict
     );
 
+    if (r < 0) {
+        radiantwave::logger::Logger::instance().warning("WiFi",
+            "RequestScan failed on device " + impl_->object_path + ": " +
+            (error.message ? error.message : strerror(-r)));
+    }
+
     sd_bus_error_free(&error);
     return r >= 0;
 }