404 error

[oliver-sanders]

Handle 404 states (e.g. viewing a workflow which doesn't exist).

Proposal to close the 404 component of #111

Cases:

• Workflow doesn't exist
  • => Display a placeholder screen - "no workflow" (#332)
  • Workflow stopped (until the UI server lists stopped workflows)
  • Workflow deleted
  • Typing error
  • Internal error
• View doesn't exist
  • => Display an error screen - "internal error"
  • Typing error
  • Internal error
• Malformed URL
  • => Display an error screen - "internal error"
  • Typing error
  • Internal error

Pull requests welcome!

[MetRonnie]

Currently we have a "Page not found" view when you navigate to a non-existent URL.

~But I think it returns HTTP 200 when it should be 404~ Actually as this is a single-page application that is irrelevant

[oliver-sanders]

Could do with a nicer 404 page too.