Make migrate process plugins more flexible/less boilerplate

I've reviewed this a couple of times today and we discussed the approach on IRC. I think this is a nice pattern and a good simplification of the skip process plugins.

I'm ambivalent about having configuration specify which method to call, but OTOH I'm all for reducing boilerplate. So +1 for this.

However...

+++ b/core/modules/migrate/src/ProcessPluginBase.php
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    // Do not call this method from children.
+    if (isset($this->configuration['method'])) {
+      return $this->{$this->configuration['method']}($value, $migrate_executable, $row, $destination_property);
+    }
+    throw new \BadMethodCallException();

The exception should include a message.

+++ b/core/modules/migrate/src/Plugin/migrate/process/SkipOnEmpty.php
+   * Skip the rest of the processing on 0.
+   */
+  public function process($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    if (!$value) {
+      throw new MigrateSkipProcessException();
+    }

The doc comment is misleading. It skips the processing if the value is falsy/empty, not just 0.

+++ b/core/modules/migrate/src/ProcessPluginBase.php
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    // Do not call this method from children.
+    if (isset($this->configuration['method'])) {

There is no check that the method actually exists and is callable. Should there be a bit of validation here, and an exception thrown if $configuration['method'] is invalid? To that end, isset() should probably be !empty().

Can we add the non-existent method name to the exception message?

Great!

Migrate is not subject beta evaluations. Committed 8d3755c and pushed to 8.0.x. Thanks!