Pipeline exception handling

[micheljansen]

It should stop processing on an unhandled exception, rather than keep going as is now the case.

[micheljansen]

Exceptions that occur are handled now. Only problem is that it's not possible to show (helpful) OpenCV errors without making the scheduler dependent on OpenCV.

[raloos]

We could introduce a OpenCVProcessor which inherits from PipelineElement to catch these exceptions automatically without making the scheduler dependent on OpenCV. Not very pretty. The alternative is to make the scheduler include opencv/cv.h which is also not very pretty, but since such a large part of the program depends on OpenCV it might be worthwhile?

[micheljansen]

I'm not sure which is better. The first one incurs extra burden on the programmer (must remember to extend the right one), the second one adds a dependency on opencv to the framework (so it will no longer compile without opencv, even if we refactor the opencvcamera and processors into plugins).