Enhance USB error handling and improve user feedback

- Updated the _friendlyErrorMessage method in UsbScreen to provide more user-friendly error messages based on specific PlatformException codes.
- Added localized error messages for various USB-related errors, improving clarity for users.
- Modified the UsbSerialService to rethrow exceptions instead of throwing StateError, allowing for better error propagation.
- Updated the usb_flow_test to reflect changes in the USB display label behavior, ensuring the test accurately describes the functionality.

lib/l10n/app_localizations_en.dart

@@ -132,6 +132,47 @@ class AppLocalizationsEn extends AppLocalizations {
   String get usbScreenEmptyState =>
       'No USB devices found. Plug one in and refresh.';
 
+  @override
+  String get usbErrorPermissionDenied => 'USB permission was denied.';
+
+  @override
+  String get usbErrorDeviceMissing =>
+      'The selected USB device is no longer available.';
+
+  @override
+  String get usbErrorInvalidPort => 'Select a valid USB device.';
+
+  @override
+  String get usbErrorBusy =>
+      'Another USB connection request is already in progress.';
+
+  @override
+  String get usbErrorNotConnected => 'No USB device is connected.';
+
+  @override
+  String get usbErrorOpenFailed => 'Failed to open the selected USB device.';
+
+  @override
+  String get usbErrorConnectFailed =>
+      'Failed to connect to the selected USB device.';
+
+  @override
+  String get usbErrorUnsupported =>
+      'USB serial is not supported on this platform.';
+
+  @override
+  String get usbErrorAlreadyActive => 'A USB connection is already active.';
+
+  @override
+  String get usbErrorNoDeviceSelected => 'No USB device was selected.';
+
+  @override
+  String get usbErrorPortClosed => 'The USB connection is not open.';
+
+  @override
+  String get usbErrorConnectTimedOut =>
+      'Timed out waiting for the device to respond.';
+
   @override
   String get scanner_scanning => 'Scanning for devices...';