dm: Improve handling of a missing uclass

When a uclass definition is missing, no drivers in that uclass can operate.
This can happen if a board has a strange collection of options (e.g. the
driver is enabled but the uclass is not).

Unfortunately this is very confusing at present. Starting up driver model
results in a -ENOENT error, which is pretty generic. Quite a big of digging
is needed to get to the root cause.

To help with this, change the error to a very strange one with no other
users in U-Boot. Also add a debug message.

Signed-off-by: Simon Glass <sjg@chromium.org>