fix(ble): handle BLEError with user-friendly messages

[lexfrei]

Summary

Replace raw Python tracebacks with helpful error messages when BLE and serial operations fail.

Before:

Traceback (most recent call last):
  File "meshtastic/__main__.py", line 1368, in common
    client = BLEInterface(...)
  ...
meshtastic.ble_interface.BLEInterface.BLEError: No Meshtastic BLE peripheral with identifier...

After:

BLE device not found.

Possible causes:
  - Bluetooth is disabled on the Meshtastic device
  - Device is in deep sleep mode
  - Device is out of range

Try:
  - Press the reset button on your device
  - Run 'meshtastic --ble-scan' to see available devices

Changes

BLE Error Handling

Wrap BLEInterface() constructor in try/except and provide context-aware error messages for:

• Device not found — BLE disabled, sleep mode, out of range
• Multiple devices found — need to specify which one
• Write errors — pairing PIN required, Linux bluetooth group
• Read errors — device disconnected unexpectedly

Serial Error Handling

Handle MeshInterface.MeshInterfaceError for connection timeouts:

• Connection timed out — device rebooting, firmware updating, serial interrupted

Test Plan

• Run meshtastic --ble nonexistent — should show "device not found" message
• Test with device in sleep mode — should show helpful suggestions
• Test on Linux without bluetooth group membership
• Test serial connection during device reboot — should show timeout message