* src/ltlvisit/randomltl.cc, src/ltlvisit/randomltl.hh: Throw
invalid_argument exceptions consistently (not std::string), and use
forwarding constructors to avoid the construct() method.
* src/bin/randltl.cc: Catch the above exceptions.  Destroy
the opts variable right after its use, so that we don't need
explicit destructor calls.
* src/ltltest/rand.test: Add a test.