CLI UI: do not print stacktrace if 'open' command is not available

[nicoloboschi]

Fix this bad stacktrace

/langstream-agents-text-processing-0.2.0-nar.nar to /tmp/nar281146335772970230/langstream-agents-text-processing-0.2.0-nar.nar.dir
Exception in thread "pool-2-thread-1" java.io.IOException: Cannot run program "open": error=2, No such file or directory
        at java.base/java.lang.ProcessBuilder.start(ProcessBuilder.java:1143)
        at java.base/java.lang.ProcessBuilder.start(ProcessBuilder.java:1073)
        at ai.langstream.cli.commands.applications.UIAppCmd.startServer(UIAppCmd.java:168)
        at ai.langstream.cli.commands.docker.LocalRunApplicationCmd.startUI(LocalRunApplicationCmd.java:385)
        at ai.langstream.cli.commands.docker.LocalRunApplicationCmd.lambda$executeOnDocker$2(LocalRunApplicationCmd.java:348)
        at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
        at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
        at java.base/java.lang.Thread.run(Thread.java:833)
Caused by: java.io.IOException: error=2, No such file or directory
        at java.base/java.lang.ProcessImpl.forkAndExec(Native Method)
        at java.base/java.lang.ProcessImpl.<init>(ProcessImpl.java:314)
        at java.base/java.lang.ProcessImpl.start(ProcessImpl.java:244)
        at java.base/java.lang.ProcessBuilder.start(ProcessBuilder.java:1110)
        ... 7 more