knox-env.sh script should exit with proper error message , if JAVA is not set.

knox-env.sh script does not exit when JAVA is not set .

Hence execution of other script (which invokes knox-env.sh to set JAVA) in an environment which does not contains JAVA , continues with execution and logs non-user friendly messages as below

Execution of gateway.sh:

nohup: invalid option -- 'j'
Try `nohup --help' for more information.

Execution of knoxcli.sh :

./knoxcli.sh: line 61: -jar: command not found