MSBuild reset error flag - msbuild

Question

I have a task that will always fail on the first call because its missing a file but I have an OnError Task which creates the file and it resumes the other tasks but the build will always fail because its in an error state. Is there anyway to reset this error state in the OnError Target?
MSDN states the following:
MSBuild executes the OnError element if one of the Target element's tasks fails with the ContinueOnError attribute set to ErrorAndStop (or false)
That being said, I am unable to execute my task if I set the ContinueOnError to WarnAndContinue (or true)
Here is my code:
<PropertyGroup>
<AppVersion>0.0.0</AppVersion>
<ChangelogFileName>Changelog_$(AppVersion).html</ChangelogFileName>
<AppCastLocation>http://test/AppCast.xml</AppCastLocation>
</PropertyGroup>
<!-- AppCastCreation target is the starting point, it attempts to download
the AppCast.xml (Target=AppCastDownload), if it fails then we create
a new AppCast.xml (Target=AppCastNew) and then update the AppCast with
the new version bneing deployed (Target=AppCastUpdate) -->
<Target Name="AppCastCreation">
<CallTarget Targets="AppCastDownload;AppCastUpdate" RunEachTargetSeparately="True" />
</Target>
<Target Name="AppCastDownload">
<!-- Download latest AppCast.xml if it doesn't exist, then create a new one -->
<WebDownload FileUri="$(AppCastLocation)" FileName="AppCast.xml" />
<OnError ExecuteTargets="AppCastNew"/>
</Target>
<Target Name="AppCastNew">
<!-- Create a new AppCast.xml -->
<Message Text="Creating new AppCast.xml" />
<Exec Command='python $(AppCastPublisherPath)AppCastPublisher.py new AppCast.xml "Changelog" "$(AppCastLocation)"'/>
<OnError ExecuteTargets="MessageErrorHandler"/>
</Target>
<Target Name="AppCastUpdate" DependsOnTargets="AppCastDownload">
<Message Text="Updating AppCast.xml with Version $(AppVersion)" />
<!-- Create changeset info and upload -->
<Exec Command="python GetJenkinsChangeset.py -html -out $(ChangelogFileName)" />
<OnError ExecuteTargets="MessageErrorHandler"/>
</Target>
So is there anyway i can reset the error state in my AppCastNew Target or potentially another workflow that would perform the same result?

Answer 1

Rather than using <OnError..., you can assign ContinueOnError the value "WarnAndContinue", and then use a condition to check the $(MSBuildLastTaskResult) property.
Old:
<WebDownload FileUri="$(AppCastLocation)" FileName="AppCast.xml" />
<OnError ExecuteTargets="AppCastNew"/>
New:
<WebDownload ContinueOnError="WarnAndContinue" FileUri="$(AppCastLocation)" FileName="AppCast.xml" />
<CallTarget Condition=" '$(MSBuildLastTaskResult)' == 'False' " Targets="AppCastNew"/>
(I believe both "WarnAndContinue" and $(MSBuildLastTaskResult) were introduced in MSBuild 4.0.)