Custom user error interface

### Discussed in https://github.com/gapitio/gapit-htmlgraphics-panel/discussions/91

<div type='discussions-op-text'>

<sup>Originally posted by **tillsteinbach** February 23, 2022</sup>
I can `throw new Error()` but it will display something like "error in onRender". Would be great if there is a real interface to show errors to the user.</div>