Provide an error message for missing dex-file arg.

Without this, veridex segfaults if the user fat-fingers the
--dex-file argument (for example by omitting the equals sign).

Test: appcompat.sh (with and without --dex-file=)
Change-Id: Ie4637c667a16864774a0bdd92a772b1b96c5ea99
1 file changed