lrdp-v2: improve error message when !vmlinux && !autodumpdir

Currently, if the user doesn't specify a vmlinux image or an
autodumpdir (like when running ramparse.py with absolutely no arguments)
they get a big ugly python stacktrace and no helpful error message. Fix
this.

Change-Id: I2d5aaaae5c8afd651ef48cfbd8f974227c2032fe