Avoid processing entities after encoding conversion failures

For https://bugzilla.gnome.org/show_bug.cgi?id=756527
and was also raised by Chromium team in the past

When we hit a convwersion failure when switching encoding
it is bestter to stop parsing there, this was treated as a
fatal error but the parser was continuing to process to extract
more errors, unfortunately that makes little sense as the data
is obviously corrupt and can potentially lead to unexpected behaviour.