CallbackManager#dispatchCallback eats exceptions

[jholloway7]

I'm using Mapbox v0.6.7 which uses MM under the hoods. I came across a weird situation where a particular zoom handler worked sometimes but not others. There was no output to the console and/or exception bubbling up. Tracing through the code I came across this:

https://github.com/stamen/modestmaps-js/blob/master/src/callbacks.js#L57

I made a quick patch to cause exceptions there to bubble up the stack and was able to figure out my logic error.

I don't currently know enough about MM to know if this is by design, but it seems there's an opportunity to do something more graceful there? Thanks.