cmd/go: improve wording of 'go mod init' error

When 'go mod init' is run without a module path, it tries to infer a
module path, based on the current directory (if in GOPATH), import
comments, and vendor configuration files.

It's common for this command to fail the first time a user tries to
create a module in a new project outside GOPATH. This change improves
the wording of the error message to hint that the user should specify
a module path.

Fixes #30678

Change-Id: Iec0352e919dbc8b426ab71eed236fad3929ec671
Reviewed-on: https://go-review.googlesource.com/c/go/+/166319Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/internal/modload/init.go

@@ -565,7 +565,7 @@ func findModulePath(dir string) (string, error) {
 		return "github.com/" + string(m[1]), nil
 	}
 
-	return "", fmt.Errorf("cannot determine module path for source directory %s (outside GOPATH, no import comments)", dir)
+	return "", fmt.Errorf("cannot determine module path for source directory %s (outside GOPATH, module path not specified)", dir)
 }
 
 var (