Please make "fn delete ..." error messages more user-friendly

[sachin-pikle]

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In both cases below I get an error, which is expected. But the message is cryptic and needs to be made more user-friendly.

1. Trying to delete an app with a function and a trigger.

$ fn delete app test-app Fn: [DELETE /apps/{app}][409] DeleteAppsApp default &{Error:0xc420449440} See 'fn --help' for more information. Client version: 0.4.150

2. Trying to delete a function with a trigger.

$ fn delete fn lisbon-app node-t-fn Fn: [DELETE /fns/{fnID}][409] DeleteFn default &{Fields: Message:Fn has associated Triggers} See 'fn --help' for more information. Client version: 0.4.150

[rdallman]

this is from swagger... attempted to do this in the last cli but it's kinda shit, have to type switch all the errors iirc. but yea, agree, we need to unroll the errors

[rikgibson]

This ties into #333 I think.