browser proxy: use one success and err handler
browser proxy: use one success and err handler

Allow setting up an extra proxy success and error handler, but don't
cache every assignment. Otherwise i.e. every onsuccess = function is
always executed instead of overriding any previously set handler.

Netsend/PerspectiveDB,Netsend/persdb,Netsend/persdb,Netsend/PerspectiveDB,Netsend/persdb,Netsend/PerspectiveDB