Start reporting missing citations and broken references in LaTeX build.

Start reporting missing citations and broken references in LaTeX build.

Now we report these in the same way as LaTeX errors (but let the user to
see the result anyway). It remains to be shown much is this disturbing
to users. Generally, ignoring these is not a good idea, because they are
harder to manually spot in longer documents.

The details of reported error varies because log linebreaks at 90
induced by pdflatex make log harder to parse.
The committed code is more robust than previous, in which some missing
cits/refs with long keys would go unnoticed.

Tested on bibtex and natbib.

https://www.mail-archive.com/lyx-devel@lists.lyx.org/msg208912.html