for example the org.apache.catalina.ant.StartTask will throw an exception
when you try to start a webapp which is allready running, this results
in a build-error in ANT. But in ANT build files where you emulate
something like hot-deploy in using a stop/startTask combination, there
are situations where you try to start an webapp which is allready running.

There should be an attribute where you can define if you really want
to throw an Exception (aka BuildError) or not. 

something like:
<tcstart  url="${catalina.url}" username="${catalina.username}" 
password="${catalina.password}" path="/maxbahr"/ builderroronfail="false">

The same goes for several other tasks like StopTask for allready stopped
webapps.

Craig mentioned correctly that a distinction between "bad-startup" or
"allready running" would be nice. IMO bad startup (which is mostly some
bad user/pw/path combination) should result in BuildError, but "allready
running" Situation should be left to the user...

I just mentioned StartTask and StopTask, there may be others where the
same pattern could be applied.