Jenkins: Turn windows build errors into warnings

Jenkinsfile

@@ -94,7 +94,16 @@ def buildDockerfile(image) {
 }
 
 def windowsBuild() {
+    stage('Windows binary') {
+        warnError('windows build failed') {
             node {
+                windowsBuildCommands()
+            }
+        }
+    }
+}
+
+def windowsBuildCommands() {
     deleteDir()
     unstash 'bin'
     docker.image('tobix/pywine').inside {
@@ -111,6 +120,5 @@ def windowsBuild() {
         archiveArtifacts '*/scripts/dist/*'
     }
 }
-}
 
 // vim: set ft=groovy: