Error handling in Sankey diagram

[hughess]

The Sankey diagram should have error handling to stop the page from crashing when an error is encountered, and to supply a helpful error message. This should cover:

• Missing props
• Incorrectly named props
• Rows where the contents of sourceCol and targetCol are the same
• Circular references

Severity

• [ ] High - I'm not able to use Evidence until this bug is fixed
• [x] Medium - I'm able to use Evidence, but a feature I need to use is not working
• [ ] Low - I'm able to keep using Evidence as normal, but flagging this for the team to fix

Expected Behavior An ErrorChart should be printed to the page.

[ud3sh]

When the dataset is empty, we can end up with undefined errors here: https://github.com/evidence-dev/evidence/blob/6baafc11f7568aedf8845809144087ce61d15fe3/packages/core-components/src/lib/unsorted/viz/sankey/SankeyDiagram.svelte#L122