Prevent loading of assets from non-asset-folders

[MeFisto94]

By throwing a newly created NotInAssetPathException.

Problems:

• How do we correctly determine that we are outside of the asset folder?
• Everyone using the Asset Manager needs to wrap that Exception and show the appropriate dialogue. We can't do that for all at once.

[MeFisto94]

https://github.com/jMonkeyEngine/sdk/blob/5be6123ca8f7f8ea4d57c23d52e61fe6713bb8e5/jme3-core/src/com/jme3/gde/core/assets/AssetDataObject.java#L182 should already be in place.