Make an error if gradle-dependencies-list is missing in the input files

The new var/gradle_dependencies_version defined in rbm.conf automatically sets a version based on the sha256sum of the file gradle-dependencies-list.txt, which is expected to be listed with the name gradle-dependencies-list in the input_files. If this is not the case, it will silently cause var/gradle_dependencies_version to be empty. This is currently the case in the build_apk step in geckoview.

Instead there should be an error when there is no input file with the name gradle-dependencies-list.