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.
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V3:
- Modify output message content
V2:
- Check return value from execvp and print message for invalid command
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Signed-off-by: Yiwei Lin <s921975628@xxxxxxxxx>
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