Gracefull failover

[llewelld]

On machines without shader capability shaderback.js will drop back to the usual static background. This happens because the gl context will be returned as null.

However, this fallback isn't very graceful as it generates a null context error in the Javascript. The user doesn't get to see this, but it would still be neater to catch the error and fail completely transparently.

The solution is to catch the error or check for a null context and turn off the relevant Javascript updates.

[llewelld]

Introduced code as of 97df0be to allow graceful failover. A debug option (set using setDebug(actiive)) allows error alerts to be generated or turned off.