Add Rethrow() method and use in BaseCacheManager

[ndrwrbgs]

Fixes #303

A little less "standard" than a wrapping exception, but satisfies the requirements without functional change beyond the stack trace. Does lose the stack trace of BaseCacheManager:L130, instead doing the Logger.LogError and then claiming to have broken control straight from wherever the exception was thrown to the outer exception handler.

Option 2 - alternative to #306