cmd/go: inaccurate go mod download errors

[marwan-at-work]

What version of Go are you using (go version)?

$ go version
go version go1.12 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

1. go mod download -json bad/path@latest
2. go mod download -json bad/[email protected]
3. go mod download -json github.com/gomods/moduledoesnotexist@latest
4. go mod download -json github.com/gomods/[email protected]

What did you expect to see?

1. Reliable code or message to say it's a bad module path
2. Reliable code or message to say it's a bad module path.
3. Reliable code or message to say that the module does not exist.
4. Reliable code or message to say that the module does not exist.

What did you see instead?

1. Incorrect error message: it should prioritize saying that the module path is incorrect rather than that the version invalid. In fact, latest is not invalid.

{
	"Path": "github.com/gomods/moduledoesnotexist",
	"Version": "latest",
	"Error": "invalid version \"latest\""
}

2. Correct error message, comparing against no.1 above, but it might be helpful to lock the message or introduce an error code.

{
	"Path": "bad/path",
	"Version": "v0.9.0",
	"Error": "unrecognized import path \"bad/path\" (import path does not begin with hostname)"
}

3. Incorrect error message same as no.1

{
	"Path": "github.com/gomods/moduledoesnotexist",
	"Version": "latest",
	"Error": "invalid version \"latest\""
}

4. Inaccurate error message: it should tell us that the Repository is not found, instead of "invalid revision"

{
	"Path": "github.com/gomods/moduledoesnotexist",
	"Version": "v0.9.0",
	"Error": "unknown revision v0.9.0"
}

cc: @jayconrod

[jayconrod]

Related #30134

[gopherbot]

Change https://golang.org/cl/189323 mentions this issue: cmd/go: improve 'go mod download' and 'go list -m' error messages