Experimental code to better handle interpreter errors.

Currently, a lot of interpreter runtime errors, such as an access to
an unset optional record field, cause Bro to abort with an internal
error. This is an experimental branch that turns such errors into
non-fatal runtime errors by internally raising exceptions. These are
caught upstream and processing continues afterwards.

For now, not many errors actually raise exceptions (the example above
does though). We'll need to go through them eventually and adapt the
current Internal() calls (and potentially others). More generally, at
some point we should cleanup the interpreter error handling (unifying
errors reported at parse- and runtime; and switching to exceptions for
all Expr/Stmt/Vals). But that's a larger change and left for later.

The main question for now is if this code is already helpful enough to
go into 2.0. It will quite likely prevent a number of crashes due to
script errors.

src/PersistenceSerializer.cc

@@ -200,7 +200,13 @@ void PersistenceSerializer::GotEvent(const char* name, double time,
 void PersistenceSerializer::GotFunctionCall(const char* name, double time,
 					Func* func, val_list* args)
 	{
-	func->Call(args);
+	try
+		{
+		func->Call(args);
+		}
+
+	catch ( InterpreterException& e )
+		{ /* Already reported. */ }
 	}
 
 void PersistenceSerializer::GotStateAccess(StateAccess* s)