Improve Gitaly connection error messages

Gitaly-ruby executes gRPC calls into gitaly server for a set of methods. Each call could fail and the error message won't give you visibility of the target endpoint it was issued for. By updating gRPC lib to a newer version the address should be included into the error as stated in the https://github.com/grpc/grpc/pull/22868 that is included into https://github.com/grpc/grpc/releases/tag/v1.30.0 There is no 1.30.0 version for the ruby client lib at https://rubygems.org/gems/grpc, that is why the latest 1.30.0.pre1 version was added.

Closes: #2786 (closed)