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_en.arb

@@ -1,4 +1,4 @@
-{
+{
   "@@locale": "en",
   "appTitle": "MeshCore Open",
   "nav_contacts": "Contacts",
@@ -53,6 +53,18 @@
   "usbScreenStatus": "Select a USB device",
   "usbScreenNote": "USB serial is active on supported Android devices and desktop platforms.",
   "usbScreenEmptyState": "No USB devices found. Plug one in and refresh.",
+  "usbErrorPermissionDenied": "USB permission was denied.",
+  "usbErrorDeviceMissing": "The selected USB device is no longer available.",
+  "usbErrorInvalidPort": "Select a valid USB device.",
+  "usbErrorBusy": "Another USB connection request is already in progress.",
+  "usbErrorNotConnected": "No USB device is connected.",
+  "usbErrorOpenFailed": "Failed to open the selected USB device.",
+  "usbErrorConnectFailed": "Failed to connect to the selected USB device.",
+  "usbErrorUnsupported": "USB serial is not supported on this platform.",
+  "usbErrorAlreadyActive": "A USB connection is already active.",
+  "usbErrorNoDeviceSelected": "No USB device was selected.",
+  "usbErrorPortClosed": "The USB connection is not open.",
+  "usbErrorConnectTimedOut": "Timed out waiting for the device to respond.",
   "scanner_scanning": "Scanning for devices...",
   "scanner_connecting": "Connecting...",
   "scanner_disconnecting": "Disconnecting...",