[Pegged] making parsing error informative

[drug]

I have the following grammar https://run.dlang.io/is/gRTGm3
If user types `ubyte1` instead of `ubyte` the whole string parsing fails 
and error message says string is wrong from the start. How can I get 
more informative message like "unknown type ubyte1" or at least error 
position points to `ubyte1` not the whole string?