Fix UnicodeDecodeError in serial_comms by adding safe fallback decodi…

[daibe]

Problem

Receiving SMS messages containing non-UTF-8 characters (e.g. Latin-1 encoded French messages) causes a crash in _readLoop:

UnicodeDecodeError: 'utf-8' codec can't decode byte ...

This leads to termination of the modem RX thread and message loss.

Root Cause

The library assumes all incoming data is UTF-8, but many GSM modems return SMS in Latin-1 or GSM encoding.

Solution

Introduce a safe decoding fallback:

• Try UTF-8
• Fallback to Latin-1
• Final fallback with ignore

Impact

• Prevents crashes in _readLoop
• Improves compatibility with real-world GSM networks
• Maintains backward compatibility

Tested on

• Huawei USB modem
• Orange / Airtel SMS (French content with accents)