Each line of the exported latex code is associated to a particular par.id()

Each line of the exported latex code is associated to a particular par.id()
of a lyx document. What par.id() is associated to the line depends on the
last call of TexRow::start() before a newline is actually issued.
If the par.id() of an ert is the one associated to the exported line,
when an error occurs, that ert is always returned by Buffer::getParFromID()
(and thus would be highlighted) whether the error actually occurred there
or elsewhere in the same paragraph.
So, better highlighting the whole paragraph when the error seems to have
occurred in ert.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@37870 a592a061-630c-0410-9148-cb99ea01b6c8