pgsql: Simplify handling of XML error messages: Just use the string

Log Message:
-----------
Simplify handling of XML error messages: Just use the string provided by
libxml as the detail message.

As per <http://archives.postgresql.org/pgsql-hackers/2006-12/msg01087.php&gt;.

For converting error codes to messages, we only need to cover those codes
that we raise ourselves now.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
xml.c (r1.20 -> r1.21)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/xml.c.diff?r1=1.20&amp;r2=1.21)
pgsql/src/test/regress/expected:
xml.out (r1.11 -> r1.12)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/xml.out.diff?r1=1.11&amp;r2=1.12)