core: Instrument relevant return paths for error tracing

core: Instrument relevant return paths for error tracing

This instruments return paths so that the origin of important errors can
be tracked down. Two so far explicit error outputs are replaced with
trace_error.

We do not instrument -ENOMEM cases unless they relate to allocations
from the remapping pool. All other -ENOMEM cases boil down to a too
small hypervisor region.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>