conditionally send partial report

[maxheld83]

In the current design, shiny sends off the rendering job in its entirety and doesn't care, nor check what happens to it. If the process dies, there's no easy way to check that from shiny.
This shortcoming is also reflected in the UI, where users will get the "successfully sent" message, even though that just means that the render process was started, not that it was completed.

this is closely related to / the basis for:

• send failure email for unexpected condition #172
• add progress indicator #88
• validate email before sending #23

The problem should affect very few users, but it should still be changed, because it also makes debugging quite hard and the whole thing feels intransparent.

However, this requires refactoring the report in some depth, and as such, should wait for crlite #296: Among other things, crlite takes care of caching at a much deeper level, so all of that complexity gets abstracted away.