prosodyctl shell: Improve process exit codes This fixes a bug introduced in 3326c8a29a86 where prosodyctl shell would always exit with non-zero error codes, even on success. Now it will exit with 0 for success, or 1 if any errors were printed. It also updates the other exit codes in the shell command to help distinguish between different error cases. These are based on the sysexits.h codes, although it is not recommended to rely on that interface.