Ensure TimeAxis w/o orientation fails cleanly

Fix minor bug where if TimeAxis is instantiated w/o an orientation, it fails
in a hard-to-decipher way because the orientation.toLowerCase() breaks on
input before the AbstractAxis class has a chance to throw a proper error.
I would rather say `Only horizontal orientation is supported`, because if it is not supported it is undefined, so there is no such thing as `horizontal Time Axis`.