tuttleHost: more comprehensible error message when bit depth are incompatible

[fabiencastan]

The Swscale plugin only supports 8 and 16 bits and the Blur node currently only supports 32 bits images.
Tuttle don't do automatic bit depth conversions, because it could deteriorate your image. So people have to choose to do this by themselves.
So if you create a graph that contains a Blur and an Swscale without any BitDepth conversion node, the process will fail.

But currently, the error message is not explicit at all. So we need to detect that case and display a comprehensible user message.

RuntimeError: libraries/tuttle/src/tuttle/host/ImageEffectNode.cpp(622): Throw in function void tuttle::host::ImageEffectNode::validBitDepthConnections() const
Dynamic exception type: boost::exception_detail::clone_impltuttle::exception::Logic
[OFX::tag_ofxStatus_] = kOfxStatErrValue
[tuttle::exception::tag_devMessage_] = Error in graph bit depth propagation.Connection between TuttleBlur_1.Source (-1 bytes) => TuttleSwscale_1.Output (1 bytes).
[tuttle::exception::tag_pluginName_] = TuttleBlur_1
[tuttle::exception::tag_backtraceMessage_] =
[tuttle::exception::tag_pluginIdentifier*] = tuttle.blur