emit better errormessage when disk queue startup fails

[rgerhards]

we should be a bit more specific, if possible, what the cause of a disk queue error is. Granted, it is inside the error code given with the message, but that's easy to overlook (even for me ;-)).

for details see #3164 (comment)

[rgerhards]

We are currently evaluating a disk queue re-write, for which we may have some sponsoring. Postponing this issue as it may be solved with that effort.