Change alluxio-masters.sh error message to STDERR

[LuQQiu]

Prerequisite:
Alluxio code convention: https://docs.alluxio.io/os/user/stable/en/contributor/Code-Conventions.html
Google shell guide: https://google.github.io/styleguide/shellguide.html

change
https://github.com/Alluxio/alluxio/blob/837a41c68b956ded56ae0fd41f0bf26b52ffbf49/bin/alluxio-masters.sh#L67

      echo "Task on '${HOSTLIST[$i]}' fails, exit code: ${ret_code}"

to

      echo "Task on '${HOSTLIST[$i]}' fails, exit code: ${ret_code}" >&2