Implement a default uncaught exception handler for REPL

[ewpatton]

We've been seeing a number of crashes due to errors in libraries and extensions doing work in other threads. We should consider implementing a global uncaught exception handler in the REPL to process these errors and report them to the user. Currently, these crashes reflect negatively on the app and we may be penalized by Google in the future even though they may originate in an extension.