JsonResponse does not deal with datetime objects; coachreports do not use standard error message display

This branch is dependent on #550 (to avoid merge conflicts)

Two issues (without issues):
* JsonResponse does not deal with datetime objects (throws an exception)
* API-driven coachreports do not use standard error message display (for example, when too many exercises are chosen to possibly display)

One additional change:
* Make coach reports API use JsonResponse object

Testing:
* Tested that code works in success and error conditions, on timeline view (which includes datetimes)

Two images before: release-0.10 code showing error (non-standard), and new (old) way showing error messages.

![screen shot 2013-08-25 at 11 43 27 am](https://f.cloud.github.com/assets/4072455/1022864/2d062e94-0db8-11e3-9baf-315e40d00dcf.png)
![screen shot 2013-08-25 at 11 42 50 am](https://f.cloud.github.com/assets/4072455/1022865/301e3a40-0db8-11e3-8293-7685c203d4a1.png)