Stdout used instead of stderr

[iguerNL]

• This behavior is noticed for models (which can be on stdout in the default setting, but we should be able to select a different std)

• The behavior is also noticed on Alt-Ergo's output on SMT files:

./alt-ergo foo.smt2 2> /dev/null 
; File "foo.smt2", line 1, characters 1-12: I don't know (0.0021) (0 steps) (goal g_1)
unknown

The line ; File "foo.smt2", line 1, characters 1-12: I don't know (0.0021) (0 steps) (goal g_1) above should be sent to stderr

[ACoquereau]

Should be fixed with #408 and #417