net: lib: dns: Fix error code returned when socket creation failed

We should return the errno value in such case, as ret will always be -1
on failure, which might misleading, as it does not give any hint on what
failed.

Also bump the log level of the error message, so the failure is visible
w/o needing to enable debug logs. Such a failure is fatal for the DNS
subsystem, so error level is justified here.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>