add better error handling on new_epdlib branch when epd library cannot be loaded

[txoof]

crashes on unknown epd -- offer better feedback to user

Traceback (most recent call last):
  File "paperpi/paperpi.py", line 658, in <module>
  File "paperpi/paperpi.py", line 620, in main
  File "paperpi/paperpi.py", line 346, in setup_display
  File "epdlib/Screen.py", line 195, in __init__
  File "epdlib/Screen.py", line 38, in new_f
  File "epdlib/Screen.py", line 264, in epd
  File "epdlib/Screen.py", line 467, in _epd_non_hd
epdlib.Screen.ScreenError: invalid waveshare module: epd2in