Simplify errors tests a bit so they don't all have to write a schema (#196)

Some of the errors tests need to have their own schema, so the schema
can do something weird (or even be entirely invalid!).  But most can
still share a schema.  In this commit I have those indeed share a
schema, to avoid having to have a bunch of copies of mostly the same
schema.  While doing so I noticed one error whose location wasn't very
useful, and fixed it.

Test plan: make check

generate/testdata/errors/schema.graphql

@@ -0,0 +1,11 @@
+type Query {
+  f: String
+  user: User
+}
+
+type User {
+  id: ID!
+  name: ValidScalar!
+}
+
+scalar ValidScalar