Add the ability to $IgnoreMissingBackupSources

- when $true, missing external drives or folders don't produce errors
- when $null or $false, missing drives and/or folders result in an error
- Default is set ot $false as not to silently fail backing up a source

config.ps1

@@ -9,6 +9,7 @@ $LogPath = Join-Path $InstallPath "logs"
 $LogRetentionDays = 30
 $InternetTestAttempts = 10
 $GlobalRetryAttempts = 4
+$IgnoreMissingBackupSources = $false
 
 # maintenance configuration
 $SnapshotMaintenanceEnabled = $true