Rework core.log:
In case of problem log with php error_log()
Introduce write2 method to better transmit log to the loggers

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>
Rework loggers to use write2 function

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>
Rework error and exception handler (catchError and catchException in AJXP_XMLWriter)

Sending too much info is bad, only send real error message to admins
or when SERVER_DEBUG is true (else send "Oops, something happened ...").
If we want to send error message to user we should do it explicitly.

If an exception is thrown in catchException, log to error_log()

AJXP_Logger now handle his exception on his own (no more try catch in catchError)

Signed-off-by: Etienne CHAMPETIER <etienne.champetier@fiducial.net>