Add a pages_generator_finalized signal.

I wrote a plugin that I'd like to also be able to run on pages in
addition to articles. Adding this signal will let me update the content
when a page is finished being generated.
fix string formatting errors
format exceptions as strings
Merge pull request #721 from mankyd/log-error

Fix String Formatting Errors
Merge pull request #719 from streeter/pages_generator_finalized

Add a pages_generator_finalized signal.