Add page context to error logging in rendering

Add page context to error logging in rendering

Add logging of the errors/warnings which rst2html outputs to its stderr
stream when rendering rst documents. Note that rst2html outputs warnings
and errors to stderr but it also adds them to the generated html. ->
hugo logs everything in stderr as error.

Add / complete adding page context (path to file being rendered) to
anything logged by getRstContent and getAsciidocContent.

See #2570