write git and git-annex version info to JSON files on commit

This in addition to ddr-local version info.
catch JSON file read errors, provide nicer error messages

With these changes, a merge conflict in a collection.json won't
bring down the collections list page, for instance.
Some error-catching in the load_json functions.
Collections and Entities objects will be instiatable but won't
contain info from JSON files.
The JSON file will contain some sort of error message, hopefully
indicating what went wrong.
Merge branch 'master' of https://github.com/densho/ddr-local