don't load unsupported segment type

[Algorush]

I suggest adding a check to aframe-streetmix-parser.js: if the segment type selected in streemix.net is not supported, don't try to load it or load the sidewalk segment instead or use the neighboring segment type? Which option is better, @kfarr ?
Since an incorrectly loaded segment can affect other functions. For example, now in this scene (https://3dstreet.app/#https://streetmix.net/kfarr/3/example-street) 3DStreet does not have support for the "drainage-channel" segment and this leads to an error:

Because of this, the boundingBox of this segment element cannot be calculated. Which leads to a problem with other functions: #449 (comment)

[kfarr]

closed by #495