"Conditional build step-single" shows nice field:
"On evaluation failure" with options "Fail the build" or mark build as unstable.

Unfortunatly this is not working.
Example 1
Run? = Never
On evaluation failure = "Mark the build unstable"
Builder = "Execute windows batch command"
Command = "echo run builder"

It should mark all builds as unstable and not run the builder.
It does not run builder but does not mark build as unstable.

Example 2 - more reliable
Run? = Execute Windows batch Command
Command = which notinpath
On evaluation failure = "Mark the build unstable"
Builder = "Execute Windows batch Command"
Command = echo builder

notinpath is not in my path, so evaluation should fail - it does.
Build should be marked as unstable - it is not.
Echo builder should not be runned - it is not.

It Shouls mark all builds as unstable - but it does not.