fix(ui): Display error message instead of DAG when DAG cannot be rendered. Fixes #3091

[alexec]

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
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Fixes #3091

Changes

• Add a panel that shows the error instead of the loading animation if the DAG cannot be loaded.
• Tidy up the summary panel:
  • Colorise phase
  • Show relative start and finish time.

Screenshots