I think the best/only way you can do this is to create a batch file to do the disconnect and have it always return a success return code. I have a similar issues with a command line app that doesn't return 0 for success and there is no way to tell Nant what return code signifies success/failure.
One of the things I do in my script is to disconnect a network drive, regardless is it's connected or not, and of course if it's not already connected I get an error. I ignore the error via failonerror=false, but I want to go one step further and suppress the error generated.
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