No general exception catching while rendering pillar

* All errors while processing pillar data are now logged besides being returned.
* Removed the general exception catch. All we need to is check for the expected data type. If it ain't, log the error and continue to the next iteration.
* Added a mocked test case which besides testing the issue reported on #5910, also tests for the proper includes data format and if salt fails accordingly and even if common data is merged.