Now the files migration does not die when a file is missing.

It now just ignore that file and continue with the rest.

web/modules/custom/geo_upgrade/src/Plugin/migrate/process/FileExists.php

@@ -0,0 +1,39 @@
+<?php
+
+
+namespace Drupal\geo_upgrade\Plugin\migrate\process;
+
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\MigrateSkipRowException;
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\Row;
+
+/**
+ * @MigrateProcessPlugin(
+ *   id = "geo_file_exists"
+ * )
+ */
+class FileExists extends ProcessPluginBase {
+
+
+  /**
+   * The file system service.
+   *
+   * @var \Drupal\Core\File\FileSystemInterface
+   */
+  protected $fileSystem;
+
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    list($source, $destination) = $value;
+
+    // Ensure the source file exists.
+    if (!file_exists($source)) {
+      throw new MigrateSkipRowException("File '$source' does not exist");
+    }
+    return $value;
+  }
+}