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.

development/Makefile.am

@@ -9,6 +9,7 @@ SUBDIRS = cygwin
 endif
 
 EXTRA_DIST = coding/Rules coding/Recommendations \
+	coverity_modeling.cpp \
 	FORMAT lyx.rpm.README \
 	lyxserver lyx.spec.in lyx.spec \
 	LyX-Mac-binary-release.sh \

development/coverity_modeling.cpp

@@ -0,0 +1,13 @@
+// This file is a modeling file for coverity
+
+namespace lyx {
+
+// Tell coverity that this function always exits
+void doAssertWithCallstack(bool value)
+{
+	if (!value) {
+		 __coverity_panic__();
+	}
+}
+
+}

src/support/convert.cpp

@@ -23,9 +23,24 @@
 
 using namespace std;
 
-namespace lyx {
+namespace {
 
-using boost::lexical_cast;
+// A version of lexical cast that does not throw. Useful for when we convert to string
+template<typename To, typename From>
+To lexical_cast(From const & value, To const & defaultResult = To())
+{
+	try {
+		return boost::lexical_cast<To>(value);
+	} catch(...) {
+		// Ignore all exceptions and use default.
+		return defaultResult;
+	}
+}
+
+}
+
+
+namespace lyx {
 
 template<>
 string convert<string>(bool b)