Make pyscal specific exceptions

[berland]

Ref. the discussion in #243 , error situations that are recognized by Pyscal should throw their own exceptions.

These exceptions should be caught by the command line client, and only the relevant message printed. Ensure that SATNUM index is included when exceptions are re-raised.

Stack-trace should not be printed by default for these error situations (unless f.ex. --debug is set) , but it should be printed for other exceptions.

Follow up this change in semeio's forward model wrapper.