pgsql: Move main error message text in plperl into errmsg from

Log Message:
-----------
Move main error message text in plperl into errmsg from errdetail,
and move the context information into errcontext instead of errmsg.
This makes them better conform to our guidelines.

Also remove a few errcode declarations that were providing the default
value ERRCODE_INTERNAL_ERROR.

Modified Files:
--------------
    pgsql/src/pl/plperl:
        plperl.c (r1.166 -> r1.167)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plperl.c?r1=1.166&r2=1.167)
    pgsql/src/pl/plperl/expected:
        plperl_init.out (r1.1 -> r1.2)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/expected/plperl_init.out?r1=1.1&r2=1.2)