Add more help to errors when linting

```
> make lint
go run tools/lint/lint.go service-manual
error parsing Markdown in service-manual/agile/features-of-agile.md:
Error at line 32, column 7 (file offset 898):
   31:
   32: Each [
            ^
```
Merge pull request #398 from alphagov/highlight-errors

Add more help to errors when linting