So you would see a copy of the string being decoded in the error message?  That string might be very long, and would be of doubtful utility.  What I had in mind was at least printing the character position of the parse failure.  That may also be of limited utility, though, since in many cases the parse failure will happen at the end of the string.  This is a problem for all parsers, and the solution is often non-trivial, depending on the parser.  

Someone who wants to see this happen will need to look at the parser and see what information it has at the point of parse failure that might be useful and practical to expose.