pgsql: Enhanced error context support in PL/Python Extract the "while

Log Message:
-----------
Enhanced error context support in PL/Python

Extract the "while creating return value" and "while modifying trigger
row" parts of some error messages into another layer of error context.
This will simplify the upcoming patch to improve data type support, but
it can stand on its own.

Modified Files:
--------------
    pgsql/src/pl/plpython:
        plpython.c (r1.126 -> r1.127)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/plpython.c?r1=1.126&r2=1.127)
    pgsql/src/pl/plpython/expected:
        plpython_record.out (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_record.out?r1=1.2&r2=1.3)
        plpython_trigger.out (r1.3 -> r1.4)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_trigger.out?r1=1.3&r2=1.4)
        plpython_types.out (r1.2 -> r1.3)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_types.out?r1=1.2&r2=1.3)
        plpython_unicode.out (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_unicode.out?r1=1.1&r2=1.2)
        plpython_unicode_2.out (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_unicode_2.out?r1=1.1&r2=1.2)
        plpython_unicode_3.out (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_unicode_3.out?r1=1.1&r2=1.2)
        plpython_void.out (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/expected/plpython_void.out?r1=1.1&r2=1.2)