ParserOutput::collectMetadata: Suppress hard failures from Parsoid

ParserOutput::collectMetadata: Suppress hard failures from Parsoid

A number of exceptions assume that the ParserAfterParse hook is called
exactly once per top-level page, and use that hook to "finalize"
various write-once properties in ParserOutput, including jsconfigvars.
Unfortunately, ParserAfterParse can't be supported properly in Parsoid
(T303630), which results in legacy extensions using this hook
overwriting extensiondata and jsconfigvars multiple times.

Instead of throwing an exception, restore the previous Parsoid
behavior where the last write wins.  This doesn't fix the root cause,
but at least it doesn't regress.  Eventually we'll have to deprecate
the ParserAfterParse hook, and when we do so we can add deprecation
warnings to these code paths in ParserOutput::collectMetadata() as
well and eventually remove them.

Noting uses ParserOutput::collectMetadata() except Parsoid at the
moment.

Bug: T303014
Bug: T303015
Change-Id: I9d1f0f6bab1305552a0350667d6142a24bc04049