Add error element to MWLiveExtensionInspector
As part of the plan to render extension errors in the
inspector instead of the view, append an element to
contain the error. More specifically, the element is
a div that contains a LabelWidget (in which the error
message is displayed) and a ButtonWidget (to expand
and collapse the error).
Initially the error element is hidden, but if Parsoid
returns an error, the element will be shown. If the
error is longer than 1 line, it is truncated but can
be expanded. On closing the inspector, any remaining
error is collapsed and hidden.
Depends on I6002d9870d2b to work