Better error messages

Give line numbers to indicate where the problem area is likely to be.
Fix markdown

After the bug fix in 258701e856.

This does mean that we can’t have:

a)
b)

etc, but those should be semantic lists rather than literals anyway.
Add test file

Slightly sucks in terms of copying the function from one file to another
to test, but highlighted a bug and let me improve the error reporting
code.
Reorder error reporting

When we have multiple things in the same file, we’d like to get errors
in sequential order, rather than reversed.
Merge pull request #384 from alphagov/fix-markdown

Fix markdown