Add Handler Validation Before Data Migration

[pocallaghan]

Preconditions

1. Use of a customised map.xml.

Steps to reproduce

1. Modify map.xml to reference a handler class that doesn't exist (e.g. <handler class="\Migration\Handler\Typo"/>).
2. Run the data migration bin/magento migrate:data -r config.xml

Expected result

1. Error before any data is copied, so you don't end up in a broken state.

Actual result

1. Error at the point it attempts to invoke that handler, this can be after a long period of time, leaving the DB in a half migrated state. Currently this leads to a very slow feedback loop to realise you've made a typo in a class name.

Additional notes

[victor-v-rad]

Hi @pocallaghan

Thank you for reporting this issue. The Data Migration Tool is an open source project and whole Magento community will welcome your contribution.