Reject when manifests are not self-descriptive

lib/ManifestLoader.ts

@@ -58,6 +58,9 @@ export class ManifestLoader {
 
     // Import all sub-manifests
     const manifest = objectLoader.resources[url];
+    if (!manifest) {
+      throw new Error(`Could not find a resource ${url} in the document at ${url}`);
+    }
     const includeJobs: Promise<void>[] = [];
     for (const includeList of manifest.properties.include) {
       for (const include of includeList.list) {

test/ManifestLoader-test.ts

@@ -28,6 +28,18 @@ const streamifyString = require('streamify-string');
 <> a mf:Manifest ;
 	rdfs:label "SPARQL 1.1 tests";
 	mf:include (<http://valid1>).
+`);
+    break;
+  case 'http://invalidroot':
+    body = streamifyString(`
+@prefix rdf:    <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
+@prefix rdfs:	<http://www.w3.org/2000/01/rdf-schema#> .
+@prefix mf:     <http://www.w3.org/2001/sw/DataAccess/tests/test-manifest#> .
+@prefix qt:     <http://www.w3.org/2001/sw/DataAccess/tests/test-query#> .
+
+<http://ex.org/abc> a mf:Manifest ;
+	rdfs:label "SPARQL 1.1 tests";
+	mf:include ("http://invalid1").
 `);
     break;
   case 'http://invalidsub1':
@@ -81,6 +93,10 @@ describe('ManifestLoader', () => {
       });
     });
 
+    it('should error on non-self describing documents', () => {
+      return expect(loader.from('http://invalidroot')).rejects.toBeTruthy();
+    });
+
     it('should return on valid submanifests', () => {
       return expect(loader.from('http://validsub1')).resolves.toEqual({
         comment: null,