Add error message before failing due to invalid dependency

If a dependency specified in DEPS is unknown or invalid (its VS
is not "git"), erlang.mk will fail. Now, add a human-readable
error message to help the user understand what the problem is.
Merge branch 'add-invalid-dep-error-msg' of git://github.com/derek121/erlang.mk
Fix relx plugin to have targets defined even if file is missing
Fail if no empty modules entry in .app.src

An empty list is required for the modules tuple, as in
{modules, []}
so a sed call could populate the list. This is mentioned in README, but
can be overlooked.

If the empty tuple is not present, relx will fail on missing dependency
errors. Therefore, fail make if the empty tuple isn't found, to
help the user diagnose the problem.