Clarify that Raven.context rethrows exceptions

The docs don't currently make it clear that an exception will be thrown again after Raven.context records it.

(This does raise a side question of whether it'd be useful to have a helper which absorbs caught exceptions to reduce the need for try / catch blocks all over the place)
Merge pull request #193 from acdha/patch-1

Clarify that Raven.context rethrows exceptions