feat: allow overriding and removing error handlers

[srivastava-diya]

Description

This PR introduces change that will allow users to override default error handlers and to remove or replace existing error handlers.
Closes : #225

Changes made

• Introduced setErrorHandler(uri, handler) which registers error handlers by URI, allowing replacement by calling it again with the same URI.
• Added removeErrorHandler(uri) for removal of a handler.
• Error handler registry signature from an array to a object keyed by URI, consistent with normalizationHandlers.

[jdesrosiers]

Looks great!