Use new ERROR_PLUGIN_ENTITY_NOT_FOUND error

[dregad]

Replacing ERROR_GENERIC will allow returning a proper 404 status code instead of 500 as we currently do..

As per @vboctor's suggestion:

Snippets/core/commands/SnippetUpdateCommand.php

Lines 78 to 80 in f1e2cac

	# TODO: ideally we should have a generic ENTITY_NOT_FOUND error to trigger 404 http status code
	# this error will trigger 500 http status code for now, it should trigge 404.
	# low priority since this is not used by the UI.

The new error code will be available in MantisbT 2.29.0 (#37316).