Fix MAGN-4015 Can not create a level is there is already a level with th...

<h6>Reviewer:</h6>
- [ ] @pboyer

<h6>Merge to</h6>
- [ ] Revit2015

When creating a level by using the Level.ByElevationAndName node, a level will be created first and then its name will be changed to the given name. But if a level with the same name already exists, the process will fail. 
Here is a picture to illustrate the case:
![image](https://cloud.githubusercontent.com/assets/5584246/4471599/3baef3c0-493c-11e4-972b-0a7ea3ce35a2.png)

The submission here will give a message more concise so that it is easier to understand why a level can not be created:
![image](https://cloud.githubusercontent.com/assets/5584246/4471533/5b71357a-493b-11e4-94bd-70bbf12f811d.png)

To implement this, all the levels will be obtained and then it will be checked whether there has already been a level with the same name, if so, we will throw an exception with the new error message.

<h6>Discussions:</h6>
@kronz 
Shall we bind the level (to be created) to the existing level if the name is the same? What if the name is the same but the elevation is different? Then shall we modify the elevation of the existing level? But this may be risky because many elements may have been created on the existing level.