Add Modal and Error modules

* Add Modal module responsible for:
  * determining which states are modal states
  * determining when to show the modal overlay
* Add Error module responsible for:
  * detecting page-wide error conditions
  * displaying information about the error
  * offering resolutions, if possible
  * showing some diagnostic information in exceptional situations
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merge master
merge issues
Merge pull request #37 from PRX/feat/error_reporting

Add Modal and Error modules