Improve helpfulness of errors

[greg]

The errors thrown by the encoder and decoder are not especially helpful for tracking down bugs in complex data models; it would help to make some of them more informative. For example:

• When attempting to encode an undecodable cycle, include the entire encoding stack in the error so the user can find the problem more easily.

• Failing USER ERROR preconditions should include the encoding path where they were triggered.

If you have any experiences with insufficiently-descriptive errors which are not listed above, please leave a comment.