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/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)