Add default error widget that can be customized (GH-48)

[jfahrenkrug]

Add a default error widget that is shown when an image fails to load. The default error widget shows a red Icons.broken_image and '🖼️💥🚫' (image, boom, no entry) as a message underneath. You can customize the error widget by subclassing EasyImageProvider and overriding errorWidgetBuilder. See the README for an example.

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Type

What kind of change does this pull request introduce?

[ ] Bug fix
[X] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other (please describe below)

Breaking Changes

Does this pull request introduce any breaking changes?

[ ] Yes
[X] No