Change from setErrorInterceptor to addErrorInterceptor

By performing this change restangular's errorInterceptor behave more like the response/requestInterceptors

I've also added tests to improve code coverage on this certain feature.

The ErrorHandler by now had the feature of preventing the Promise to fail (by returning false). This feature is still available (hence, full backwards compatibility) - if one of all error handlers returns false the promise will not be rejected.