Improve feedback from squelched exceptions

[sjmiles]

https://github.com/Polymer/observe-js/blob/master/src/observe.js#L367

There is more information available on ex then console.error emits to the console.

Changing it to something like this:

        console.error('Exception caught during observer callback: ', ex.stack || ex);

Gives a lot more useful information, on Chrome at least.

I didn't make a pull request because I didn't do any due diligence other than throw that change into my local checkout. I don't know what this does on any other browser, e.g., and I suspect Arv knows a lot about this topic.

@arv: btw, SD Polyfill has a similar issue where it rethrows a squelched error which loses the stack information. It could use the same treatment.

[sjmiles]

Fwiw, this seems to work as well on Chrome and might be better across platforms (only difference is one argument to console.error instead of two):

 console.error('Exception caught during observer callback: ' + (ex.stack || ex));