Fix unserializer error handling function

Problem/Motivation

The webhook_process_unserializer_error() function doesn't pass in the name of the failing plugin, and handles the data to log differently than webhook_process_processor_error().

Steps to reproduce

Call a webhook without providing any data to unserialize.

Proposed resolution

Refactor webhook_process_unserializer_error() to work like webhook_process_processor_error().

Remaining tasks

Patch forthcoming.

User interface changes

N/A

API changes

N/A

Data model changes

N/A