scripts/decode_stacktrace: warn when modpath is needed but is unset

When a user tries to parse a symbol located inside a module he must have
modpath set. Otherwise, decode_stacktrace won't be able to parse the
symbol correctly.

Right now the failure is silent and easily missed by the user. What's
worse is that by the time the user realizes what happened (or someone on
LKML asks him to add the modpath and re-run), he might have already got
rid of the vmlinux/modules.

Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>