Exception hierarchy cleanup

[allen0099]

Exception hierarchy cleanup for 0.3.8. All changes are additive; removing CacheError is tracked for 0.4.0.

Split into one sub-issue per pull request:

• Export CacheXError, BackendNotFoundError and RequestNotFoundError from the package #160
• Raise a dedicated ProxyNotSetError from manager proxies #161
• Derive SessionError from CacheXError #162
• Deprecate the unused CacheError #163

[allen0099]

All four sub-issues are done (#160, #161, #162, #163). Removing the deprecated CacheError is tracked in #130 for 0.4.0.