UnicodeDecodeError when restoring from eml files

Unfortunately I don't think I can provide the email that failed because the import seems to be unordered and I don't believe gyp logs the file being processed before this point.

Wrapping this in error handling, logging the error and filename, and continuing with the restore would be a nice improvement.

```
Traceback (most recent call last):
File "/opt/homebrew/bin/gyb", line 2897, in
main(sys.argv[1:])
~~~~^^^^^^^^^^^^^^
File "/opt/homebrew/bin/gyb", line 2517, in main
message = mbox.next()
File "/opt/homebrew/Cellar/gyb/1.82_1/libexec/lib/python3.13/site-packages/fmbox.py", line 94, in next
msg.set_from(self._last_from_line[5:].decode())
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xa3 in position 70: invalid start byte
```

Start by locating fmbox.py and the restore path around mbox.next(), which raises the UnicodeDecodeError shown in the report. Reproduce the failure with an .eml containing the problematic byte; done means the filename and decoding error are logged and the restore continues.

Written by the indexing model from the issue text.