EventArgs for BindingFailed must match Log.Warning

[spadapet]

Description of Change

This is the follow-up for the initial XAML BindingDiagnostics PR by @StephaneDelcroix:
#10523

See the bug I logged:
#12645

The change I'm making allows binding failures to show up properly in VS's binding failures pane:

• The Message string didn't always exactly match the debug output for the binding failure
• In order to de-dupe the BindingFailed event against debug output, I need the failure messages to exactly match. This change ensures that the messages must match

Issues Resolved

• fixes [Bug] BindingDiagnostics.BindingFailed message doesn't always match debug output #12645

API Changes

Changed:

• BindingBaseErrorEventArgs.ErrorCode gets a value of "Binding" or "MultiBinding" depending on what gets written to debug output (instead of not ever having a value). VS will need to know that in order to reconstruct the failure message and de-dupe against debug output.

Platforms Affected

• Core/XAML (all platforms)

Behavioral/Visual Changes

None

Before/After Screenshots

Not applicable

Testing Procedure

Made all necessary changes in VS to handle the BindingFailed event from Xamarin. Manually made sure that Android and UWP debugging shows binding failures in the new XAML Binding Failures pane. Tested that double click will "go to code".

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[rmarinho]

Unit test failing

https://dev.azure.com/xamarin/public/_build/results?buildId=29664&view=ms.vss-test-web.build-test-results-tab

[spadapet]

Unit test failing

https://dev.azure.com/xamarin/public/_build/results?buildId=29664&view=ms.vss-test-web.build-test-results-tab

Thanks I'll make a fix.