Previously, when we hit a parse error, we recorded the sequence and
position in the exception, which is all you need to know to find the
location, but is pretty frustrating to use in practice.

Therefore, when the sequence is a string, figure out the line and column
number (1-based, to match the numbers reported by text-editors), and put
that in the exception message. It's not perfect, not least because it
doesn't mention a filename, but it's better than what we had before.