More tests for web handlers (#2755)

* Return negative values of Total and Free in StorageInfo() when we fail to get disk info

* Return consistent messages in web handlers when the server is not initialized

cmd/xl-v1.go

@@ -224,6 +224,12 @@ func (xl xlObjects) StorageInfo() StorageInfo {
 	// Sort so that the first element is the smallest.
 	sort.Sort(byDiskTotal(disksInfo))
 
+	if len(disksInfo) == 0 {
+		return StorageInfo{
+			Total: -1,
+			Free:  -1,
+		}
+	}
 	// Return calculated storage info, choose the lowest Total and
 	// Free as the total aggregated values. Total capacity is always
 	// the multiple of smallest disk among the disk list.