Bug fix: Fix errorWidget on dart DDC (web)

[JustusFluegel]

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Bug fix: The errorWidget would never be displayed when running using DDC (dart web) and the HttpGet option, which happens because the catch block currently never gets executed when using streams with await for on dart DDC. As such this pr refactors the file to use a StreamController instead, re-implementing the original behavior.

:arrow_heading_down: What is the current behavior?

Placeholder stays displayed even if a error response (e.g. 404) happened, when running using DDC

:boom: Does this PR introduce a breaking change?

No, this is a pure bug fix.

:bug: Recommendations for testing

Maybe add a DDC test suite? idk

:memo: Links to relevant issues/docs

• https://github.com/dart-lang/sdk/issues/47764

:thinking: Checklist before submitting

• [x] All projects build
• [x] Follows style guide lines (code style guide)
• [x] Relevant documentation was updated
• [x] Rebased onto current develop