Promote Framework not found (MSB3644) warning to error

When the targeted .NET framework reference assembly folder is not found,
an error (rather than warning) will be generated. This will prevent
producing a build that could be invalid.

Closes dotnet#173

src/XMakeTasks/GetReferenceAssemblyPaths.cs

@@ -284,7 +284,7 @@ private IList<String> GetPaths(string rootPath, FrameworkNameVersioning framewor
             // No reference assembly paths could be found, log a warning as there could be future errors which may be confusing because of this.
             if (pathsToReturn.Count == 0)
             {
-                Log.LogWarningWithCodeFromResources("GetReferenceAssemblyPaths.NoReferenceAssemblyDirectoryFound", frameworkmoniker.ToString());
+                Log.LogErrorWithCodeFromResources("GetReferenceAssemblyPaths.NoReferenceAssemblyDirectoryFound", frameworkmoniker.ToString());
             }
 
             return pathsToReturn;