[flutter_svg] Implement errorBuilder callback

[memishood]

closes flutter/flutter#158612

Description

• Implemented errorBuilder callback to handle loading failures

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.
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• I linked to at least one issue that this PR fixes in the description above.
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• 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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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing. The test exemption team is a small volunteer group, so all reviewers should feel empowered to ask for tests, without delegating that responsibility entirely to the test exemption group.

[domesticmouse]

Hey @memishood, thank you for your Pull Request.

Can you please add a test to confirm that this code works as expected?

[domesticmouse]

Please add tests

[memishood]

Please add tests

Alright, I will be adding them soon

[domesticmouse]

LGTM, PTAL @stuartmorgan