allow middleware to catch any exception

as part of #1541 and #1549 middleware support for catching exceptions was added, but the interface to it is unwieldy as it requires all exceptions to subclass a specific base exception .. given use of any library code or larger applications this is rather difficult to accomodate in practice. Given we have a default global error handler that does the exact same thing as the view response, i think uncaught exceptions (minus chalice view error) should propagate by default so middleware has a chance to handle it, and the global error acts as a fallback.

Review the existing middleware exception-catching path and the default global error handler described in the issue. Trace how uncaught exceptions and Chalice view errors are currently routed, then verify that middleware can handle general exceptions while the global error handler remains the fallback.

Written by the indexing model from the issue text.