Make DB Import backwards compatible

closes #2716
- Continues to parse and allow import with the old format
- Detects the presence of the new JSON-API format and allows import
Add distinct error classes

closes #2690
- added new error classes
- moved errorhandling.js to /errors/index.js
- changed API errors to use new classes
- updated tests
Merge pull request #2704 from sebgie/issue#2690

Add distinct error classes
Merge pull request #2719 from appleYaks/import-backward-compatible

Make DB Import backwards compatible
Add try-catch to startup

- Should prevent Ghost from exiting without an error message