Fix coverity issues about exceptions

Fix coverity issues about exceptions

There a some exceptions related to the fact that BOOST_ASSERT throws
an unhandled exception, which is fait enough. This is handled by
uploading a modeling file to coverity.

The second batch of issues are related to the use of lexical_cast in
convert.cpp. We use now a wrapper around boost::lexical_cast that does
not throw but return empty strings instead. I am not sure actually of
when lexical_cast could fail.