spl: fit: be more verbose when an error occurs when applying the overlays

spl: fit: be more verbose when an error occurs when applying the overlays

There are many ways the overlay application can fail.
2 of them are probably the most common:
- the application itself failed. Usually this is comes from an unresolved
  reference
- DTBO not available in FIT (could be because of a typo)

In both case it is good to be more explicit about the error and at least
show which overlay is failing.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>