[camera_web] Add camera errors handling

[bselwe]

Adds camera errors handling to the web implementation of the camera platform interface.

• Added media camera error codes to CameraErrorCode (media error).
• Added CameraWebException thrown when the camera reports an initialization, configuration or video streaming error, or enters into an unexpected state.
• Moved getMediaStreamForOptions to CameraSettings - returns a media stream for the camera constrained by the given options.
• Added camera errors handling in the camera plugin.
  • Methods will now throw a PlatformException rather than a DomException/CameraException for consistency with the MethodChannelCamera implementation. The only exemptions are availableCameras and createCamera that may throw a CameraException.
  • Each CameraWebException is caught and emitted on the onCameraError stream.
  • Added camera video element's onError and onAbort events to 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.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[bselwe]

@ditman @felangel @orestesgaolin

[ditman]

Let's keep the momentum going. Since CameraSettings is an internal implementation detail, it can be renamed/split in a further PR.

[ditman]

Is it time to find a better name for CameraSettings? Is it maybe doing too much? Slightly confusing naming with CameraOptions?

Maybe the parts regarding the management of the Camera Stream should be moved to a new class? Is this class more of a CameraStreamsHelper/Handler?

[bselwe]

Good point! It is currently responsible for getting the camera stream based on CameraOptions, getting the facing mode setting based on the camera video track and mapping camera video track settings. How about renaming it to a CameraService (this way we will no longer mix settings with options)?

I'm also okay with extracting the camera stream logic to a separate class (CameraStreams.getStreamForOptions maybe?) if you prefer it that way.

[ditman]

CameraService sounds good to me. For the conversion functions, what I've been doing is to have a fat "convert.dart" with a bunch of functions there that translate from one type to my own (and vice-versa). See convert.dart in google maps. I normally prefer those functions to be stateless, so they can be just utility functions (they don't need to belong to any class or namespace).