skip_on_substr plugin doesn't support logging messages to migration logs

Problem/Motivation

When a row or process is skipped after using the `skip_on_substr` plugin, there is currently no capability to log anything to the migration logs;

$ drush migrate:import migration_abc
[notice] Processed 463 items (461 created, 0 updated, 0 failed, 2 ignored) - done with migration_abc
$ drush migrate:message migration_abc
[notice] No messages for this migration

This can be problematic as it's not always clear which entry was skipped, or why.

Proposed resolution

Extend the `skip_on_substr` plugin to support a `message` config item and logging the value to the migration logs