Don't throw if a doc hasn't been versioned yet (#455)

Instead of throwing, return `null` because that means we have a
new doc in our versioning sequence

(Also, cleaned up a bit of code as I researched this)

Partial Fix To #450

lib/server/versionFallback.js

@@ -133,9 +133,7 @@ files.forEach(file => {
 // what the requested version is
 function docVersion(id, req_version) {
   if (!available[id]) {
-    throw new Error(
-      `Document with id '${id}' was requested but no document with that id could be located.`
-    );
+    return null;
   }
   // iterate through versions until a version less than or equal to the requested
   // is found, then check if that version has an available file to use