Standardise the reporting of rendering errors

[jamieoliver]

The new macro syntax added by #51 introduces a number of places where rendering errors can occur and need to be reported cleanly without affecting rendering of the rest of the page. Ideally this behaviour should be standardised across the whole wiki so errors occurring from other parts of the rendering stack can be similarly reported.

[dometto]

What about collecting errors in the metadata under the 'errors' key, as the YAML filter is now doing?