Add content-type header to grpc error locations

[ciarams87]

Proposed changes

The gRPC Go client shows errors when handling error pages returned from the IC (e.g. when a backend is unavailable). This resolves one of the issues (malformed header response), e.g.:

Current error:
./grpc_client -address virtual-server.example.com:443 2021/11/30 14:32:41 could not greet: rpc error: code = Unknown desc = unexpected HTTP status code received from server: 204 (No Content); malformed header: missing HTTP content-type

After this commit:
./grpc_client -address virtual-server.example.com:443 2021/11/30 14:21:05 could not greet: rpc error: code = Unknown desc = unexpected HTTP status code received from server: 204 (No Content)

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto master
• I will ensure my PR is targeting the master branch and pulling from my branch from my own fork

[pleshakov]

hm, in my tests, the response I got when there are no available endpoints:

2021/11/30 17:22:34 could not greet: rpc error: code = Unavailable desc = unavailable

which looks good.