Fixed bugs with lists as input. Added tests for the exceptions

bob/learn/em/isv_trainer.cpp

bob/learn/em/ivector_trainer.cpp

bob/learn/em/jfa_trainer.cpp

bob/learn/em/plda_trainer.cpp

bob/learn/em/test/test_ivector_trainer.py

bob/learn/em/test/test_jfa_trainer.py

bob/learn/em/test/test_plda_trainer.py