fix: Fix torch state sync issues

[navaronbracke]

This PR fixes a bug where the torch state was not properly synced with the notifier.
Android, iOS and MacOS use callbacks to notify when their observable torch state has changed.
However, the plugin also manually set the torch state, which made it go out-of-sync.

Changelog:

• Instead of throwing an error during toggleTorch, if the device is null, do nothing
• Remove some redundant print() calls, instead adding a comment why exceptions are ignored
• Only turn the toch on for iOS/MacOS when requested and if the torch value would change the current torch state
• Bring the iOS/MacOS toggle torch implementations closer together, while also checking the isTorchAvailable besides hasTorch for iOS. This is some work towards making Use sharedDarwinSource to share MacOS and iOS implementation details #782 easier.
• Use error.localisedDescription as message for FlutterError, rather than putting it in the code field
• On MacOS, like iOS, also continue the capture if the torch could not be turned on, rather than throwing an error
• Replace a usage of containsKey() with a nullable value access in the web plugin. This reads a bit better, and provides type promotion for the variable.
• Fixed a bug where the torch state was not set to TorchState.off when the camera is stopped
• Fixed a bug where the torch state notifier has its value updated outside of the torch observer callback

The last two fixes listed here probably fixed the out-of-sync issue of #833

Fixes #833