Add build-step check for proto submodules

[XAMPPRocky]

Currently if you download and run cargo build you get an unintuitive error message about missing proto files, before building the protobuf in build.rs we should check if the modules are present, and tell people to run git submodule update --init first.

[markmandel]

Should probably also add to our docs here as well https://googleforgames.github.io/quilkin/main/book/installation.html

[markmandel]

@XAMPPRocky we can close this now, since we vendor and generate protos at develop time, not build time?