Instead of calling PLy_elog() for reporting Python argument parsing
errors, generate appropriate exceptions. This matches the existing plpy
functions and is more consistent with the behavior of the Python
argument parsing routines.
In commit 5c3c3cd0a3, the new tests were
apparently just dumped into the first convenient file. Move them to a
separate file dedicated to testing that functionality and leave the
plpython_test test to test basic functionality, as it did before.
