More strict use of errbacks when using deferreds.

More strict use of errbacks when using deferreds.

Now almost all calls to addCallback include adding an errback as well.
HTTPDownloader will remove the next request on a connection error.
HTTPServer will download the whole file after a rendering error.
A failed check of the freshness of a file will cause the whole file to be downloaded.
A resolving error in a DHT join behaves as if the bootstrap node was unreachable.
Removed the errback args to khashmir's find routines as they were never used.