bin/cassandra should complain if $JAVA is empty or not an executable

If JAVA_HOME points to a non-existing directory, bin/cassandra just complains with [node1 ERROR] /Users/snazy/.ccm/test/node1/bin/cassandra: line 151: exec: : not found (via ccm).

Patch checks if $JAVA is empty or does not point to an executable and prints appropriate error message.

(Patch against trunk)