abort build when an unresolved dependency is found (#2696)

TestAssets/NonRestoredTestProjects/TestProjectWithUnresolvedDependency/Program.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace ConsoleApplication
+{
+    public class Program
+    {
+        public static void Main(string[] args)
+        {
+            Console.WriteLine("Hello World!");
+        }
+    }
+}

TestAssets/NonRestoredTestProjects/TestProjectWithUnresolvedDependency/project.json

@@ -0,0 +1,18 @@
+{
+  "version": "1.0.0-*",
+  "compilationOptions": {
+    "emitEntryPoint": true
+  },
+  "dependencies": {
+    "Microsoft.NETCore.App": {
+      "type": "platform",
+      "version": "1.0.0-rc2-*"
+    },
+    "ThisIsNotARealDependencyAndIfSomeoneGoesAndAddsAProjectWithThisNameIWillFindThemAndPunishThem": { "target": "project" }
+  },
+  "frameworks": {
+    "netcoreapp1.0": {
+      "imports": "dnxcore50"
+    }
+  }
+}