securesync api app setup, fixing retrypurgatory reporting bug, pulling out translatable strings

Just a bunch of small changes:
* the main api sub-app has api_{models,views,urls}; the securesync api sub-app had api_{models,views}, but urls were in securesync/urls.py.  I moved them to securesync/api_urls.py and lined them from securesync/urls.py as done in the main code.

* When retrypurgatory failed, it wrote its message to stdout instead of stderr.  Thus, errors in this chronograph job went undetected.  I output a localizable error (with a bit of debugging instruction) to stderr now.

* Many strings in syncmodes.py and retrypurgatory.py were not localizable.  I made them localizable.

This has been tested with a multi-facility central-distributed setup.