dim: Improve the error output when rebuild-nightly fails

Joonas rightly complained that the current output is useless and just
confuses.

v2: Bikeshed stuff some more.

v3: Split out misplaced hunk.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>