CLC-3291 Quick fix for Textbooks so it won't cache JSON derivations of error states

JIRA: https://jira.ets.berkeley.edu/jira/browse/CLC-3291

This is a quick fix that only addresses the Textbooks feed. Some of the other proxies (basically all those that aren't part of the Live Update set) also have the same bug, but I don't have time to fix them this sprint because it involves a general refactoring of that proxy line. 

This commit also fixes ProxyError to handle a common use case where proxies pass in a log message without a response body. This change makes the returned response what you would expect from those cases.