server: Convert all non-status errors to codes.Unknown (#1881)

- convertCode utilized errors that were not allowed by the library per 9d0bc30edb/doc/statuscodes.md
- Relevant issue: #1672

stream.go

@@ -36,7 +36,10 @@ import (
 )
 
 // StreamHandler defines the handler called by gRPC server to complete the
-// execution of a streaming RPC.
+// execution of a streaming RPC. If a StreamHandler returns an error, it
+// should be produced by the status package, or else gRPC will use
+// codes.Unknown as the status code and err.Error() as the status message
+// of the RPC.
 type StreamHandler func(srv interface{}, stream ServerStream) error
 
 // StreamDesc represents a streaming RPC service's method specification.