socketcan: Increase RX buffer size to 20MB

RX buffer size has been underestimated. The size of the kernel's sk_buff
induces a high overhead compared to the 16B of a CAN packet. Increase
the kernel buffer to have enough margin in case of momentary traffic spikes.
socketcan: add *stream when throwing DashelException

When throwing a DashelException, a valid stream should be provided.
Otherwise Dashel Hub::step will fail on assert, aborting the
application.
Merge pull request #283 from vaussard/fix-mobovero-can

Fix mobovero can (v2)