sd-bus: always fill in sd_bus_error paramters, on error

sd-bus: always fill in sd_bus_error paramters, on error

Whenever one of our calls is invoked with a non-NULL, writable
sd_bus_error parameter, let's fill in some valid error on failure. We
previously only filled in remote errors, but never local errors, which is
hard to handle by users. Hence, let's clean this up to always fill in
the error.

This introduces a new bus_assert_return() macro that works like
assert_return() but optionally also initializes a bus_error struct.

Fixes #224.

Based on a patch by Umut Tezduyar.