cpupower: monitor: Exit with error status if execvp() fail

cpupower: monitor: Exit with error status if execvp() fail

In the case that we give a invalid command to idle_monitor for
monitoring, the execvp() will fail and thus go to the next line.
As a result, we'll see two differnt monitoring output. For
example, running `cpupower monitor -i 5 invalidcmd` which `invalidcmd`
is not executable.

Link: https://lore.kernel.org/r/20250220163846.2765-1-s921975628@gmail.com
Signed-off-by: Yiwei Lin <s921975628@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>