Move Tracker's exception logging code to a place where Blaze can call it

[mattmccutchen]

To fix meteor/blaze#273, Blaze's exception logging code needs the same change that was made to Tracker's exception logging code in meteor/meteor#3655. Copying and pasting the code from Tracker to Blaze does not seem like a good solution; this probably won't be the last time the code will need to be modified to deal with browser issues.

Is there a place I can move the exception logging code where it can be called from Blaze? How about packages/meteor/errors.js? Is it OK for Tracker to call a function defined in that file? (Are the comments about Tracker working without Meteor still valid?) Anything else I should be aware of before spending the time to prepare a pull request?

This can be considered part of #95. (Maybe most of it?)