Raise a dedicated ProxyNotSetError from manager proxies

[allen0099]

Part of #113.

ProxyBase.get() raises BackendNotFoundError for every proxy, including CacheManagerProxy, SessionManagerProxy and StateManagerProxy ("No instance set for proxy ..."). The name is misleading when no backend is involved.

Proposal

Add ProxyNotSetError(BackendNotFoundError) and raise it from the manager proxies. Subclassing keeps existing except BackendNotFoundError clauses working.