Low coverage in grpc-json transcoder filter

grpc-json transcoder filter currently has lower than normal coverage due to untested error handling paths.

The prot to JSON formatter in the update protobuf library removed the limitation on the depth of nesting in the Struct proto that can be converted to JSON. A test that validated this case was removed as a result. Unfortunately it was also testing the error handling path on the encoding path, which caused the coverage to dip 93.8%

This issue tracks addition of tests that validate error handling in JSON to proto and proto to JSON transcoding.