Add enhanced parser error output

[harningt]

The parser should return some sort of details about how parsing failed.

For example, if trailing commas are denied, the string "[true, ]" should return the fact that trailing commas are denied and the position that this was detected. Position should probably be represented as line-number/character-within-line. That way a file could reasonably be scanned for the parsing error for correction.

The error string should be both machine-parseable and human-readable. The 'next' tree has current work on this, but the format for errors is not yet well-defined.