Description:
When a missing mixin is encountered for a-sphere, WebApp crashes. However, for a-entity with line, the missing mixin doesn't trigger crash.

Either one of the behaviour could be valid, but needs to be consistent.

• A-Frame Version: 1.0.4
• Platform / Device: Win10/Firefox
• Reproducible Code Snippet or URL:

   <a-entity id="axes">
   		<a-entity id="origin_line" line="start: 0, 0, 0; end: 20000 0    0; color: red;   opacity:1" mixin="missing_mixin_no_error"><a-entity>
   		<!--a-sphere mixin="missing_mixin_throws_error"-->	</a-sphere><!--uncomment to trigger crash-->
   	</a-entity>