vndk-def: Add command line option checks.

This commit adds path existence checks to --system and --vendor command
line options.  The error message will make it easier for users to
figure out what's wrong with the output.

Bug: b/32811412
Test: ./vndk_definition_tool.py vndk --system [non-existing-dir]
Change-Id: Ia910f88781fa76803909921ae1be2cfcfb002b6d