fix error handling in Set-TNCertificate

[potatoqualitee]

[7s] C:/github/ansible❯ Set-TNCertificate @certparams Set-SFTPItem : File already exists on remote host. At line:198 char:29

• ... $null = Set-SFTPItem -Destination /tmp -Path $CaCertPath

• 
  + CategoryInfo          : InvalidOperation: (SSH.SftpSession:SftpSession) [Set-SFTPItem], SftpPermissionDeniedException
  + FullyQualifiedErrorId : File already exists on remote host,SSH.SetSftpItem

WARNING: [07:58:02][Invoke-SecureShellCommand] Command ' /opt/sc/support/bin/php /opt/sc/src/tools/installCA.php /tmp/custom_CA.inc' failed with exit status 106