A missing entry point should be reported early

[tautschnig]

At present, goto_symext::symex_from_entry_point_of will throw an exception if no entry point is found. I believe the existence of an entry point should become a PRECONDITION and instead the command-line front-ends invoking symex should perform this check.

The reason for this concern is that errors should be as early as they can possibly be detected. Delayed error reporting wastes users' time.