Warnings from MSBuild Tasks crash BCC-MSBuildLog

[jkoritzinsky]

If a warning is issued by the <Warning> MSBuild task, then BCC-MSBuildLog will crash when parsing that warning since it is coming from a file that isn't a subdirectory of the source directory.

The warning should either be ignored, or parsed and put into a general section that isn't tied to specific files.

[jkoritzinsky]

See https://dev.azure.com/AvaloniaUI/AvaloniaUI/_build/results?buildId=1071&view=logs for a repro (i.e. this occurs from built-in MSBuild tasks)

[jkoritzinsky]

It seems like the Xamarin.iOS "connection does not exist" warnings aren't correctly handled. (They should probably be ignored since they're likely always going to happen on CI builds and aren't an issue.)