Better error handling

[Rhys Evans]

4 tiers

1. parsing query structure - need to investigate peg.js api for this. Maybe use https://github.com/rse/pegjs-util
2. parsing query params - can probably do a lot better than currently
3. data fetching errors - map to more friendly messages
4. post processing errors - if part 3 is done then this shoudl be easier to do