Scratch-gui: Detect and show friendly error message if WebGL is not supported

[thisandagain]

Expected Behavior

• GUI should present a friendly error message if WebGL is not supported

Actual Behavior

• Fatal exception and stage fails to load

Steps to Reproduce

• Use computer or browser without WebGL support
• Load https://llk.github.io/scratch-gui

[thisandagain]

/cc @carljbowman

[thisandagain]

@carljbowman Can we get a design for this that is similar to the IE warning?
/cc @chrisgarrity @cwillisf

[carljbowman]

Mock (v2)

Follows the same pattern as https://github.com/LLK/scratch-gui/issues/828 and https://github.com/LLK/scratch-gui/issues/982

Version 1 - Space Jam Ipsum

Content

Illustration

Copy

• [x] Header: TBD - @thisandagain

Your Browser Does Not Support WebGL

• [x] Copy: TBD - @thisandagain

Unfortunately it looks like your browser or computer [does not support WebGL](https://en.wikipedia.org/wiki/WebGL#Support). This technology is needed for Scratch 3.0 to run.

Button

Back - Should call a browser Back command

Secondary Copy

Footer: To learn more, go to the Preview FAQ.

• FAQ URLscratch.mit.edu/preview-faq

[jalman]

@carljbowman I think there's a typo. The word "is" is missing between "Space Jam" and "one".

By the way, Space Jam is my favorite movie too! It's truly incomparable.

[carljbowman]

@jalman - This _is_ exactly why I should not write copy.

[thisandagain]

Boring non-space-jam themed copy has been added. 🏀 👾

[carljbowman]

@chrisgarrity - Just added an illustration to my comment above.