Add clearer error text a few places (#117)

## Summary:
It's nice to be clear here because it may be your first interaction with
genqlient!

## Test plan:
make check


Author: benjaminjkraft

Reviewers: StevenACoffman, benjaminjkraft, dnerdy

Required Reviewers: 

Approved By: StevenACoffman

Checks: ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint, ✅ Test (1.17), ✅ Test (1.16), ✅ Test (1.15), ✅ Test (1.14), ✅ Lint

Pull Request URL: https://github.com/Khan/genqlient/pull/117

generate/config.go

@@ -117,8 +117,8 @@ func initConfig(filename string) error {
 	}
 	w, err := os.OpenFile(filename, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0o644)
 	if err != nil {
-		return err
+		return errorf(nil, "unable to write default genqlient.yaml: %v", err)
 	}
 	_, err = io.Copy(w, r)
-	return err
+	return errorf(nil, "unable to write default genqlient.yaml: %v", err)
 }