Option to serve stale content for a cached_include in case of errors.

[kazekent]

TODO:

• Possible to check if sub-templates have been served with cache? Want to avoid inadvertently storing bad results in long-term cache at higher level, but can't throw the exceptions up the stack.

[kazekent]

context.errors.empty? won't suffice if cached includes further down the chain has swallowed any errors...