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/ConflictingDirectiveArguments.schema.graphql

@@ -1,3 +0,0 @@
-type Query {
-  f: String
-}

generate/testdata/errors/ConflictingDirectives.schema.graphql

@@ -1,3 +0,0 @@
-type Query {
-  f: String
-}

generate/testdata/errors/ConflictingTypeNameAndForFieldBind.schema.graphql

@@ -1,8 +0,0 @@
-type Query {
-  user: User
-}
-
-type User {
-  id: ID!
-  name: String!
-}

generate/testdata/errors/ConflictingTypeNameAndLocalBind.schema.graphql

@@ -1,8 +0,0 @@
-type Query {
-  user: User
-}
-
-type User {
-  id: ID!
-  name: String!
-}

generate/testdata/errors/InvalidQuery.schema.graphql

@@ -1,3 +0,0 @@
-type Query {
-  f: String
-}

generate/testdata/errors/NoQuery.schema.graphql

@@ -1 +0,0 @@
-type Query { f: String }

generate/testdata/errors/schema.graphql

@@ -1,4 +1,5 @@
 type Query {
+  f: String
   user: User
 }
 

generate/testdata/snapshots/TestGenerateErrors-ConflictingTypeNameAndGlobalBind-graphql

@@ -1 +1 @@
-testdata/errors/ConflictingTypeNameAndGlobalBind.schema.graphql:8: typename option conflicts with global binding for ValidScalar; use `bind: "-"` to override it
+testdata/errors/ConflictingTypeNameAndGlobalBind.graphql:4: typename option conflicts with global binding for ValidScalar; use `bind: "-"` to override it