feat: throw for reference not found

This change makes the distinction between schema problems and validation
of the instance errors.

Author: kylef

Sources/Core/ref.swift

@@ -1,12 +1,14 @@
+enum ReferenceError: Error {
+  case notFound
+}
+
 func ref(context: Context, reference: Any, instance: Any, schema: [String: Any]) throws -> AnySequence<ValidationError> {
   guard let reference = reference as? String else {
     return AnySequence(EmptyCollection())
   }
 
   guard let document = context.resolve(ref: reference) else {
-    return AnySequence([
-      ValidationError("Reference not found '\(reference)'", instanceLocation: nil),
-    ])
+    throw ReferenceError.notFound
   }
 
   let id: String?

Tests/JSONSchemaTests/JSONSchemaTests.swift

@@ -167,4 +167,18 @@ class ValidateTests: XCTestCase {
       XCTAssertEqual(error.instanceLocation?.path, "/0")
     }
   }
+
+  func testReferenceNotFound() {
+    let schema: [String: Any] = [
+      "$ref": "#/unknown",
+    ]
+
+    do {
+      _ = try validate("anything", schema: schema)
+    } catch {
+      return
+    }
+
+    XCTFail("Validation did not throw error")
+  }
 }