Error if user follows a non-existent component

[itowlson]

Fixes #944.

If a non-existent component is specified in --follow, Spin now exits with this message:

$ spin up --follow godby --follow helo
Error: The following component(s) specified in --follow do not exist in the application:
  - godby
  - helo
The following components exist:
  - gogogo
  - goodbyte
  - hello
Error: exit status: 1

(I don't know where the "exit status 1" comes from - is that an artefact of the spin trigger command failing and that output being reported by spin up @lann?)

(cc @etehtsea - it won't let me add you as a reviewer)

Signed-off-by: itowlson ivan.towlson@fermyon.com

[etehtsea]

Suggested change

	let unknown_names: Vec<_> = names
	.iter()
	.filter(|name| !component_ids.contains(name.as_str()))
	.collect();
	let unknown_names: Vec<_> = names.difference(&component_ids).collect();

[itowlson]

Bah, I knew that had to exist somewhere. Thank you!

[etehtsea]

I've checked it locally, and it works great!
Thanks!

[lann]

Almost equivalent (but with extra trailing \n):

Suggested change

	.map(|item| format!(" - {item}"))
	.collect::<Vec<_>>()
	.join("\n")
	.map(|item| format!(" - {item}\n"))
	.collect()

[itowlson]

That's nice, but because I return the error message rather than fully controlling the printing it means, I end up with a blank line between the error and the exit code message:

$ spin up --follow godby --follow helo
Error: The following component(s) specified in --follow do not exist in the application:
  - helo
  - godby
The following components exist:
  - gogogo
  - hello
  - goodbyte

Error: exit status: 1

...which is neither here nor there, I'm sure, but I slightly prefer the more compact form. If we can get rid of the exit status message then this would be a good change. (And thanks for mentioning it, I didn't know collecting to String did this. Though now I do I'm not sure how I feel about it!)