lrdp-v2: debug_image_v2: Make NotImplementedErrors less noisy

Currently, if cache dumping is not supported on a given target, we put
up a traceback and error out.  The traceback is ugly and not helpful.
Catch the NotImplementedError and print a nicer message.

Change-Id: I07d1e3a5b56b809135d41919bcc992931764a0af