fix(scan): warning if lsof command not found

[kotakanbe]

What did you implement:

[Feb  6 08:27:21] DEBUG [c7] Executing... lsof -i -P -n | grep LISTEN                                                                        
[Feb  6 08:27:21] DEBUG [c7] execResult: servername: c7                                                                                                       
cmd: /usr/bin/ssh -tt -o StrictHostKeyChecking=yes -o LogLevel=quiet -o ConnectionAttempts=3 -o ConnectTimeout=10 -o ControlMaster=auto -o ControlPath=/home/ubuntu/.vuls/controlmaster-%r-c7.%p -o Controlpersist=10m [email protected] -p 22 -i /home/ubuntu/.ssh/stg.pem -o PasswordAuthentication=no stty cols 1000; sudo -S lsof -i -P -n | grep LISTEN                                                                                                                     
exitstatus: 1                                                                                                                                               
stdout: sudo: lsof: command not found                                                                                                                                                                                                                                                                                   
stderr:                                                                                                                                                     err: %!s(<nil>)                                                                                                                                           

#863 changed to debug message when grep count is zero, and debug message when lsof command is not found(It was error before).

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

^C⏎                                                                                                                                                                                                                                                                                                                                                                                                    ✗  ubuntu@dev  ~│g│s│g│f│vuls  ⎇ lsof-err-handle~  ./vuls scan c7
[Feb  7 07:20:00] ERROR Failed to create log file. path: /var/log/vuls/localhost.log, err: open /var/log/vuls/localhost.log: permission denied
[Feb  7 07:20:00]  INFO [localhost] vuls-v0.15.6-build-20210207_070958_cd67220
[Feb  7 07:20:00]  INFO [localhost] Start scanning
[Feb  7 07:20:00]  INFO [localhost] config: /home/ubuntu/go/src/github.com/future-architect/vuls/config.toml
[Feb  7 07:20:00]  INFO [localhost] Validating config...
[Feb  7 07:20:00]  INFO [localhost] Detecting Server/Container OS... 
[Feb  7 07:20:00]  INFO [localhost] Detecting OS of servers... 
[Feb  7 07:20:00] ERROR Failed to create log file. path: /var/log/vuls/c7.log, err: open /var/log/vuls/c7.log: permission denied
[Feb  7 07:20:00]  INFO [c7] vuls-v0.15.6-build-20210207_070958_cd67220
[Feb  7 07:20:00]  INFO [localhost] (1/1) Detected: c7: centos 7.7.1908
[Feb  7 07:20:00]  INFO [localhost] Detecting OS of containers... 
[Feb  7 07:20:00]  INFO [localhost] Checking Scan Modes... 
[Feb  7 07:20:00]  INFO [localhost] Detecting Platforms... 
[Feb  7 07:20:01]  INFO [localhost] (1/1) c7 is running on aws
[Feb  7 07:20:01]  INFO [localhost] Detecting IPS identifiers... 
[Feb  7 07:20:01]  INFO [localhost] (1/1) c7 has 0 IPS integration
[Feb  7 07:20:01]  INFO [c7] Scanning OS pkg in fast-root mode
[Feb  7 07:20:08]  WARN [c7] Failed to lsof: Failed to lsof: execResult: servername: c7
      cmd: /usr/bin/ssh -tt -o StrictHostKeyChecking=yes -o LogLevel=quiet -o ConnectionAttempts=3 -o ConnectTimeout=10 -o ControlMaster=auto -o ControlPath=/home/ubuntu/.vuls/controlmaster-%r-c7.%p -o Controlpersist=10m [email protected] -p 22 -i /home/ubuntu/.ssh/stg.pem -o PasswordAuthentication=no stty cols 1000; sudo -S lsof -i -P -n
      exitstatus: 1
      stdout: sudo: lsof: command not found
    
      stderr: 
      err: %!s(<nil>):
    github.com/future-architect/vuls/scan.(*base).lsOfListen
        /home/ubuntu/go/src/github.com/future-architect/vuls/scan/base.go:900
[Feb  7 07:20:14]  INFO [c7] Scanning listen port...


Scan Summary
================
c7      centos7.7.1908  303 installed, 134 updatable

Checklist:

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES