Seriously improve error reporting in DownloadThread.

My old error reporting strategy for DownloadThread was to log the
stack trace for the exception, so we'd know exactly what conditions
caused the StopRequest.  hackbod suggested that we shouldn't log
tracebacks as they clutter the log.  Instead, we should just always
include a little string tag explaining why the request is being
stopped -- this is more concise and more useful to developers.

There are three main changes here to acheive this goal:
* make StopRequest require a short, log-friendly error message upon
  construction, and add such a message to all construction sites
* make a similar change to GenerateSaveFileError, so that the variety
  of errors that originate with Helpers.generateSaveFile() get
  similarly fine-grained and concise error reporting
* make network usable checking code return a distinct error code for
  each distinct negative condition, and add a utility to return a
  log-friendly error message for each such code.

Finally, I cleaned up some of the ways errors/exceptions are handled
in the process.

Change-Id: Ie70cbf3f2960e260e97f8449258e25218d0f900f

0 files changed, 0 insertions, 0 deletions