Parsing errors in reStructuredText files have no source

[pekkaklarck]

If there's a parsing error in reST file, the reported error looks like this:

Error in file 'None' on line 4: Non-existing setting 'Bad Setting'.

Luckily fixing the source is easy.

Another problem is that the line number isn't the actual line in the source file but a line in the parsed code block. Fixing that would be a lot harder and I don't consider it worth the effort at least now.