Add a plugin / world setting for missing resource parameter panicking, skipping, and warning

[fallible-algebra]

0.15.1 reverted the behaviour of "non-Option resources aren't present" from its 0.15 behaviour (non-panicking, warn once, skip the system for as long as those resources aren't present) to 0.14 behaviour (panic). While there now exists a method to allow this warn-once behaviour per-system, there is not a global / plugin setting to change this behaviour.

What problem does this solve or what need does it fill?

Eliding the need to migrate already-existing large projects to this new behaviour.

What solution would you like?

A plugin or world setting to re-enable "warn once, skip, and don't panic" behaviour as the "default" for new systems.

What alternative(s) have you considered?

• Extending App with some kind of RegisterSkippableSystems trait that registers systems and marks them as warn-once don't-panic, and using this exclusively.
• Reverting my own project to 0.15.

Additional context

I began working on my own project during the 0.15 release candidate process with "don't run if the resources aren't present" as a behaviour in mind, it being a welcome change to me in early prototyping. While I could migrate to this new fallible systems method of doing things (and I likely will at a later date – but at the moment I'm focused on other parts of development), this will involve a lot of rewriting and the system parameters for systems I write in future will be more verbose.

Related issues: #16578, #15265, #17138, #17178, #17324

[fallible-algebra]

It seems that trying to avoid 0.15.1 and depend on =0.15.0 is a fragile alternative due to this change happening in a "patch" semver version. edit: But adding bevy_ecs = "=0.15.0" does actually fix the dependencies at 0.15.0.