* Work around commonjs protoc bug
When compiling with the commonjs target, the protoc compiler still emits
references to Closure Compiler-isms that whack global state onto the
global object. This is particularly bad for us since we expect to be
able to make backwards-compatible changes to our Protobuf definitions
without breaking things, and this bug makes it impossible to do so.
To remedy the bug, this commit hacks the output of protoc (again) with
sed in order to avoid ever touching the global object. Everything still
works fine because the commonjs target (correctly) exports the protobuf
message types via the module system - it's just not writing to global
anymore.
* Fix status.proto
* Don't hack status.proto
* Send structured errors across RPC boundaries
This brings us closer to gRPC best practices where we send structured
errors with error codes across RPC endpoints. The new "rpcerrors"
package can wrap errors from RPC endpoints, so RPC servers can attach
some additional context as to why a request failed.
* Code review feedback:
1. Rename rpcerrors -> rpcerror, better package name
2. Rename RPCError -> Error, RPCErrorCause -> ErrorCause, names
suggested by gometalinter to improve their package-qualified names
3. Fix import organization in rpcerror.go
