Changed logic of entrypoint in Lambda4

- removed exception Fail
- added "check",which checks if we are in a subproblem
  of separation on which we know to be complete
- "solve" does not raise particular exceptions anymore,
  and the program raise failure only if there is some bug

(cherry picked from commit 608c40045f651c6402b17c437f997de4d63f6afd)