set_error_handler(null) and set_exception_handler(null) now return the
previous error/exception handler instead of just returning bool(true).
This is consistent with the behavior of these functions with non-null
values.
This allows the error handler to be reset using set_error_handler(null).
As the code suggests this behavior was already previously intended, but
the callback check was done too strictly.
This reverts the following two commits:
* 6ba2e662e4
* d8f8e98d8e
Laruence already did some partial changes to set_error_handler and
set_exception_handler. I'm reverting those modifications to apply the full
set of changes. (The modifications changed the code structure in a way that
would lead to more duplication with the new behavior.)
