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: #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)
 }

generate/generate.go

@@ -316,7 +316,8 @@ func Generate(config *Config) (map[string][]byte, error) {
 	// way.  (As-is, we generate a broken file, with just (unused) imports.)
 	if len(document.Operations) == 0 {
 		// Hard to have a position when there are no operations :(
-		return nil, errorf(nil, "no queries found, looked in: %v",
+		return nil, errorf(nil,
+			"no queries found, looked in: %v (configure this in genqlient.yaml)",
 			strings.Join(config.Operations, ", "))
 	}
 

generate/testdata/snapshots/TestGenerateErrors-NoQuery-go

@@ -1 +1 @@
-no queries found, looked in: testdata/errors/NoQuery.go
+no queries found, looked in: testdata/errors/NoQuery.go (configure this in genqlient.yaml)

generate/testdata/snapshots/TestGenerateErrors-NoQuery-graphql

@@ -1 +1 @@
-no queries found, looked in: testdata/errors/NoQuery.graphql
+no queries found, looked in: testdata/errors/NoQuery.graphql (configure this in genqlient.yaml)