Pass parsoid server errors through to VisualEditor.

Unfortunately the Http::get wapper provides no data if anything but
an OK status is returned, so ApiVisualEditor now uses MWHttpRequest
directly.

If a non-OK status in encountered the error code and message are
passed through to the JSON output, and rendered in the error
alert by VE.

The error code passed through is parsoidserver-http-NNN although
other suggestions are welcome.

Bug: 44354
Change-Id: I166ecf81c40f0f1c62663f2096753269d28f2916