[camera_web] Handle camera errors in takePicture

[bselwe]

Added handling of camera errors in takePicture. The method may throw a CameraWebException after changes introduced in #4222. The exception is handled by throwing a PlatformException and emitting a camera error event on the onCameraError stream.

Part of flutter/flutter#45297.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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[bselwe]

@ditman @felangel

[ditman]

Let's go!