Provide somewhat more meaningful parser errors.

While still a bit cryptic this is probably as best as we can get it. An example:

    Oga.parse_xml("<namefoo:bar=\"10\"")

    parser.rb:116:in `on_error': Unexpected string on line 1: (Racc::ParseError)

    => 1: <namefoo:bar="10"

This fixes #43.