[flutter_svg] Error handling enhancement

[minhhung2556]

Purpose

To fix these related issues below

• Exception: Invalid image data on preCacheImages dnfield/flutter_svg#1100
• Error found in FirebaseCrashlytics dnfield/flutter_svg#1085
• Correct error handling in custom ByteLoader dnfield/flutter_svg#1080
• Load a picture in case the given picture doesn't exists. dnfield/flutter_svg#1044
• Add errorBuilder to SvgPicture and adjust underlying vector_graphics errorBuilder (to trigger on BytesLoader errors) dnfield/flutter_svg#996
• Catching error or having an "error" placeholder dnfield/flutter_svg#1007

My related PRs

• Error handling enhancement dnfield/flutter_svg#1104
• Error handling enhancement dnfield/vector_graphics#258
• [flutter_svg] Error handling enhancement #8016

Changes Made

• Catch error cases and return results to SVGPicture.
• Expose errorWidgetBuilder.
• Write more unit tests.
• Update example.

Testing

• Already tested by example and unit test in flutter_svg

Impact

• This change should not affect other parts of the library.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to 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, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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