After upgrading to the latest Jenkins LTS & durable task plugin (v1.21) our pipelines fail on warnings.

Pipeline:

node() {
    bat 'C:\\Python27\\python.exe -c "import warnings;warnings.warn(\'future\', FutureWarning)"'

    powershell 'C:\\Python27\\python.exe -c "import warnings;warnings.warn(\'future\', FutureWarning)"'
}

Output:
2018-03-08 19:47:13 [src] Running batch script
2018-03-08 19:47:14
2018-03-08 19:47:14 d:\src>C:\Python27\python.exe -c "import warnings;warnings.warn('future', FutureWarning)"
2018-03-08 19:47:17 -c:1: FutureWarning: future
[Pipeline] powershell
2018-03-08 19:47:18 [src] Running PowerShell script
2018-03-08 19:47:20 powershell.exe : -c:1: FutureWarning: future
2018-03-08 19:47:20 At D:\src@tmp\durable-4343e8b5\powershellWrapper.ps1:3 char:1
2018-03-08 19:47:20 + & powershell -NoProfile -NonInteractive -ExecutionPolicy Bypass -File ...
2018-03-08 19:47:20 + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2018-03-08 19:47:20 + CategoryInfo : NotSpecified: (-c:1: FutureWarning: future:Strin g) [], RemoteException
2018-03-08 19:47:20 + FullyQualifiedErrorId : NativeCommandError
2018-03-08 19:47:20

This returns in "1" exit code, causing the pipeline to fail.

So somehow, somewhere, the warning is transformed / interpreted as an error.