Clean up "download()" implementation

Reduce nesting, remove repetition.

lib/storage-handler.js

@@ -233,16 +233,44 @@ exports.download = function(provider, req, res, container, file, cb) {
 
   var range = null;
 
-  if (req) {
+  if (!req) {
+    // TODO(rfeng/bajtos) We should let the caller now about the problem!
+    return;
+  }
+
   if (req.headers) {
     range = req.headers.range || '';
   }
 
-    if (range) {
+  if (!range) {
+    return download(params);
+  }
+
   provider.getFile(params.container, params.remote, function(err, stats) {
     if (err) {
+      return handleError(res, err);
+    }
+
+    setupPartialDownload(params, stats, res);
+    download(params);
+  });
+
+  function download(params) {
+    var reader = provider.download(params);
+
+    res.type(fileName);
+
+    reader.pipe(res);
+    reader.on('error', function onReaderError(err) {
       handleError(res, err);
-        } else {
+    });
+    reader.on('end', function onReaderEnd() {
+      cb();
+    });
+  }
+};
+
+function setupPartialDownload(params, stats, res) {
   var total = stats.size;
 
   var parts = range.replace(/bytes=/, '').split('-');
@@ -258,32 +286,4 @@ exports.download = function(provider, req, res, container, file, cb) {
   res.set('Content-Range', 'bytes ' + params.start + '-' + params.end + '/' + total);
   res.set('Accept-Ranges', 'bytes');
   res.set('Content-Length', chunksize);
-
-          var reader = provider.download(params);
-
-          res.type(fileName);
-
-          reader.pipe(res);
-          reader.on('error', function(err) {
-            handleError(res, err);
-          });
-          reader.on('end', function() {
-            cb();
-          });
-        }
-      });
-    } else {
-      var reader = provider.download(params);
-
-      res.type(fileName);
-
-      reader.pipe(res);
-      reader.on('error', function(err) {
-        handleError(res, err);
-      });
-      reader.on('end', function() {
-        cb();
-      });
-    }
-  }
 };