Provide a way for users to format their own error messages

[agladysh]

Current implementation produces error message of hundreds of lines for larger grammars.

Ideally as a nearley user I would be able to format the error message myself from the data on the Error object.

One option to do that could be to add all necessary data as fields on the Error object, and add .toString() to it, formatting the error message as it is done now.