Log more detail when "Block structure does not match" in feed

[mhasself]

In ocs_feed.py, Block object raises exceptions if "Block structure does not match", in the append/extend methods. However, no detail is provided in these (such as which keys are offending). Since it's often hardware failure / reconnection issues that lead to this happening, and such situations are difficult to efficiently (re-)produce, it would be helpful to increase verbosity in those error messages!