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