Implement error handling

**Why**: Sub of #1028
**What**: If you put a manifest file in that doesn't quite conform, then it doesn't really give you an error. We need to at least return some error, or ideally some information to help them understand how to resolve.

**Scope of work:**
- [ ] Display an error message if a manifest file doesn't quite conform to the structure , if a specific error is possible, display that, otherwise a general message with a link to [IF docs](https://if.greensoftware.foundation/intro).