Tidy up error reporting when converting PL/Python arrays.

Tidy up error reporting when converting PL/Python arrays.

Use PLy_elog() only when a call to a Python C API function failed, and
ereport() for other errors. Add an error code to the "wrong length of
inner sequence" ereport().

Reviewed-by: Daniel Gustafsson
Discussion: https://www.postgresql.org/message-id/B8B72889-D6D7-48FF-B782-D670A6CA4D37%40yesql.se