if --aptafile doesn't exist, there's no error handler and reading throws a json format error instnead

[laxris]

To reproduce

chris@notebook:~/FlexFringe$ ./flexfringe --ini=ini/alergia.ini --mode=predict --predictalign=1 data/staminadata/1_training.txt.dat --aptafile=path/to/datafile.ff.final.json

Using heuristic alergia
Creating apta using evaluation class alergia
predict mode selected
reading apta file - path/to/datafile.ff.final.json
terminate called after throwing an instance of 'nlohmann::detail::parse_error'
  what():  [json.exception.parse_error.101] parse error at 1: syntax error - unexpected end of input; expected '[', '{', or a literal

Loguru caught a signal:....