Error conditions don't get to decide their own human-readable string

It *must* be derivable from the (textual) error code and data, in order
to support translation.

For now, just use the error code as the human-format text.
Merge branch 'update-dc-client-wip' of github.com:socrata/soda-fountain into update-dc-client-wip
Errors are objects!

Will need to do this for the data-coordinator too.  It'll be good for
translation, having all the errors in one place.