jconsole.sh should fail fast if JBOSS_HOME is incorrect instead of starting the console with an incorrecdt classpath

If $JBOSS_HOME is incorrectly set, we get a vague WARNING message when the script starts, but jconsole is run and shows the GUI window. This is misleading, because jconsole won't have the correct classpath and won't be able to connect to Wildfly, leading to an ambiguous "Cannot connect" error.

jconsole.sh should fail fast if it cannot find the jboss-cli-client.jar and avoid this entire confusion.

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Steps to reproduce:

1. export JBOSS_HOME=/no/existing/path
2. ./standalone.sh