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/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, ", "))
 	}