introduce error management when updating render graph

[mockersf]

fixes #1069

while investigating this issue, it seems to be:

• creating a new window is done through an event that is treated asynchronously, at the beginning of a frame
• updating the render graph to use it is done synchronously in the current frame

I tried fixing it by calling handle_create_window_events more often or at the appropriate time, but it's hard due to the fact it has a event_loop param.

The other way to fix this is to make the render graph more error resilient.

To do this, the update function of a node now returns a Result<(),()>, and the graph will just ignore errors. Similarly, the various panic! calls in the loop have been replaced by a warning! and a break to continue to the next node.

If this approach to fix the issue is taken, there is still to do:

• the warning messages are useless as the user can't really do anything about them
• the Result<(),()> type should use at least a real error type
• there may be a few cases where panic! is actually a better behaviour, I'm not sure how to identify them

[mockersf]

ha clippy actually doesn't like Result<(),()>... nice clippy!

[mockersf]

I would rather restart this from scratch if I need it again than finish this PR