Possible to produce an appropriate error when decoding messages larger than 64 MiB?

Hi, thanks for creating this project! It looks like https://github.com/dart-lang/protobuf/blob/cdffe5e612ef0ab7243858a9127e58edef719c98/protobuf/lib/src/protobuf/coded_buffer_reader.dart#L9 causes messages larger than 64 MiB to be silently truncated, resulting in a generic `InvalidProtocolBufferException`. May I ask the reason behind doing so? The 64 MiB magic number cannot be found from any public documentation; I think throwing an exception which explicitly indicates that the message is too big could make this easier to debug.