Fail manifest2proto when imports are missing (#5932)
Fail manifest2proto when imports are missing (#5932)

Previously running the `arcs_manifest_proto` BUILD rule on a manifest file that imports a non-existent manifest file would print an error but would not fail the build. This makes it fail correctly.
PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs,PolymerLabs/arcs