Too big of runtime

[ericmjonas]

If a user creates a runtime that unpacks to be too large, make sure we deliver sane errors to the user.

[ericmjonas]

In particular the wrenhandler tries to download the pickled objects like data and the function before it deals with the handler. So this is a problem for multiple reasons

• If func or data is too large, we will error
• if there is left-over crap from a previous run, we will error

[ericmjonas]

A reasonable workaround if the runtime ate up too much space on the last invocation can be to call map with use_cached_runtime=False

[ericmjonas]

Closed by PR #172