grpc-rust/protoc-rust-grpc/src/lib.rs should not panic, or at least parametrize

[przygienda]

it would be nice grpc-rust/protoc-rust-grpc/src/lib.rs returns a proper error on missing compiler, wrong version etc. instead of panic. That would on different platforms just throw a warning from build.rs when the code is e.g. precompiled & checked in. Not much work, if necessary can do that ...

[stepancheg]

Patch welcome.