Call internal format "internal", not "modern".  Enforce it more.

* onetime:
  (Format_Level): Change to "internal".  Replace "modern" with
    "internal" wherever appropriate in code and comments.
  (PadDecoder.decode): Undo exception-wrapping of commit e62d3104ec5.
    Instead we will catch all expected exception types in the caller,
    because Python 2.x doesn't have true exception wrapping.
  (Pad._handle_inner_header): Enforce values in first flag byte.
  (main): Adjust for exception changes in PadDecoder.decode.

* check.sh: Adjust for above exception change.