Nexus: Polish error copy around when to include parents in API selectors

[david-crespo]

repo search for "should not be specified"

It's a small thing, but I want to clean up the grammar, punctuation, clarity, and consistency on these errors that are very easy to run into when first learning how to use the API. There are a bunch. Here are just a few:

omicron/nexus/src/app/internet_gateway.rs

Lines 55 to 60 in c140817

	} => Err(Error::invalid_request(
	"when providing gateway as an ID vpc and project should not be specified",
	)),
	_ => Err(Error::invalid_request(
	"gateway should either be an ID or vpc should be specified",
	)),

omicron/nexus/src/app/disk.rs

Lines 59 to 64 in c140817

	} => Err(Error::invalid_request(
	"when providing disk as an ID project should not be specified",
	)),
	_ => Err(Error::invalid_request(
	"disk should either be UUID or project should be specified",
	)),