[webview_flutter] Added a toString() method to the NSError class

[SourishBiswas23]

This PR is adding a toString() method in the NSError class so that all of its information can be printed. This was recommended by the author of the issue that this PR resolves. Issue: flutter/flutter#128596

The issue number 128596 in the flutter repository is being fixed by this PR

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. (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 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, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• 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.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[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 to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

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.

[SourishBiswas23]

The change in this package is minor and doesn't affect the compatibility of this package. Please add the label 'override: no versioning needed'.

[stuartmorgan-g]

Please add the label 'override: no versioning needed'.

Which of the exemptions in the linked versioning policy are you requesting this under?

[SourishBiswas23]

Please add the label 'override: no versioning needed'.

Which of the exemptions in the linked versioning policy are you requesting this under?

Sorry. It was a mistake on my part. I misread one of the conditions.

[cyanglaz]

Code looks good, could you add a test for it?

[stuartmorgan-g]

@SourishBiswas23 Are you still planning on updating this PR to address the feedback above?

[Hixie]

@SourishBiswas23 Thanks for your contribution. I made a PR similar to yours (#4441) that does adds tests and does the changelog stuff, so I'm going to close this one. Thanks again!