aoe: make error messages more specific in static minor allocation

For some special-purpose systems where udev isn't present, static
allocation of minor numbers is desirable.  This update distinguishes
different failure scenarios, to help the user understand what went
wrong.

Signed-off-by: Ed Cashin <ecashin@coraid.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>