Add handling for missing diff directory files

[R3ality]

Resolution for issue #15

[R3ality]

Example code

$ref = "C:\temp\IntuneBackup-20200520_112127Z"
$dif = "C:\temp\IntuneBackup-20200520_113136Z"
Compare-IntuneBackupDirectories -ReferenceDirectory $ref -DifferenceDirectory $dif

Example output

MISSING FILE: Reference directory file 'C:\temp\IntuneBackup-20200520_112127Z\Device Configurations\Test-DeviceConfigurationProfile-Restrictions.json' is not
 found in the difference directory.
MISSING FILE: Reference directory file 'C:\temp\IntuneBackup-20200520_112127Z\Device Configurations\Assignments\Test-DeviceConfigurationProfile-Restrictions.
json' is not found in the difference directory.