XWIKI-9750: User Input in WYSIWYG Editor is lost after conversion error
* Use the output from the ConversionFilter in wysiwyginput.vm after a conversion error.
* Send the conversion error back instead of redirecting to edit mode in case the request is AJAX (e.g. Save & Continue)
XWIKI-9750: User Input in WYSIWYG Editor is lost after conversion error

 * backport fix to 5.0.x-branch, need to code it without
   the escapetool.url(Map) method, which is new in 5.2