Fix: Avoid UnknownPropertyException in validation error response

[marc-farre]

Prevents the following error message and should allow getValidationErrorResponse() to return a proper response instead of throwing an error.

yii\base\UnknownPropertyException: Getting unknown property:
humhub\modules\cfiles\models\Folder::baseFile in
/var/www/protected/vendor/yiisoft/yii2/base/Component.php:154
Stack trace:
#0
/var/www/protected/vendor/yiisoft/yii2/db/BaseActiveRecord.php(296):
yii\base\Component->__get('baseFile')
#1
/var/www/protected/humhub/modules/content/components/ContentActiveRecord.php(210):
yii\db\BaseActiveRecord->__get('baseFile')
#2
/var/www/protected/modules/cfiles/actions/UploadZipAction.php(40):
humhub\modules\content\components\ContentActiveRecord->__get('baseFile')
#3
/var/www/protected/modules/cfiles/actions/UploadZipAction.php(32):
humhub\modules\cfiles\actions\UploadZipAction->getValidationErrorResponse(Object(humhub\modules\cfiles\models\Folder))
#4
/var/www/protected/humhub/modules/file/actions/UploadAction.php(72):
humhub\modules\cfiles\actions\UploadZipAction->handleFileUpload(Object(yii\web\UploadedFile),
false)
#5
/var/www/protected/modules/cfiles/actions/UploadAction.php(36):
humhub\modules\file\actions\UploadAction->run()
#6 [internal function]:
humhub\modules\cfiles\actions\UploadAction->run()
#7
/var/www/protected/vendor/yiisoft/yii2/base/Action.php(93):
call_user_func_array(Array, Array)
#8
/var/www/protected/vendor/yiisoft/yii2/base/Controller.php(178):
yii\base\Action->runWithParams(Array)
#9
/var/www/protected/vendor/yiisoft/yii2/base/Module.php(552):
yii\base\Controller->runAction('upload', Array)
#10
/var/www/protected/vendor/yiisoft/yii2/web/Application.php(103):
yii\base\Module->runAction('cfiles/zip/uplo...', Array)
#11
/var/www/protected/vendor/yiisoft/yii2/base/Application.php(384):
yii\web\Application->handleRequest(Object(humhub\components\Request))
#12
/var/www/index.php(27)

[marc-farre]

@luke- PR not tested because I don't know how to reproduce the bug, but should work.