Give a more graceful error message if the lock file is broken

Author: jasonmalinowski

src/Microsoft.NuGet.Build.Tasks/ResolveNuGetPackageAssets.cs

@@ -885,6 +885,11 @@ private IEnumerable<NuGetPackageObject> GetPackagesFromTarget(JObject lockFile,
 
                 Func<string> fullPackagePathGenerator;
 
+                if (libraryObject == null)
+                {
+                    throw new ExceptionFromResource(nameof(Strings.MissingPackageInTargetsSection), package.Key);
+                }
+
                 // If this is a project then we need to figure out it's relative output path
                 if ((string)libraryObject["type"] == "project")
                 {

src/Microsoft.NuGet.Build.Tasks/Strings.Designer.cs

@@ -132,6 +132,9 @@
   <data name="MissingMSBuildPathInProjectPackage" xml:space="preserve">
     <value>Your project is consuming assets from the project '{0}' but no MSBuild project is found in the project.lock.json. Check the project references in your project file, and re-run NuGet restore.</value>
   </data>
+  <data name="MissingPackageInTargetsSection" xml:space="preserve">
+    <value>The package '{0}' could not be found in the libraries section of the lock file. This may indicate your lock file is corrupted.</value>
+  </data>
   <data name="MissingProjectReference" xml:space="preserve">
     <value>The project.json is referencing the project '{0}', but an output path was not specified on an item in the {1} property.</value>
   </data>