Bug 20272: Replace error numbers by codes in XSLT_Handler

Bug 20272: Replace error numbers by codes in XSLT_Handler

We remove the error numbers 1 to 7 by readable codes.
And remove the errstr attribute (not used widely).
Make XSLT_Handler a little bit less noisy by defaulting print_warns to
false unless $ENV{DEBUG} is set. (See also bug 19018).

The unit has been changed accordingly.
(A few warnings are no longer tested.)

Test plan:
Run t/db_dependent/XSLT_Handler.t

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>