dissect: avoid clobbering device-mapper error when activating verity

dissect: avoid clobbering device-mapper error when activating verity

The device-mapper driver can return a wild variety of errors when trying
to activate the same dm-verity volume concurrently, as it might happen
with an image. There is a fallback logic in place, but the original
return code was clobbered when userspace signature check was added.
Add it back.

Follow-up for c2fa92e7e8907d9