Better messages and return codes in applications

Use stderr and return codes >0 when it makes sense.
Loose return code conventions:
* 1 for generic errors
* 2 for user input errors
* 3 for remote errors (not found, etc.)
* 4 for not implemented

weboob/applications/havesex/havesex.py

@@ -133,7 +133,7 @@ class HaveSex(ReplApplication):
     def edit_optims(self, backend_names, optims_names, stop=False):
         if optims_names is None:
             print >>sys.stderr, 'Error: missing parameters.'
-            return 1
+            return 2
 
         for optim_name in optims_names.split():
             backends_optims = {}
@@ -165,7 +165,7 @@ class HaveSex(ReplApplication):
     def optims(self, function, backend_names, optims, store=True):
         if optims is None:
             print >>sys.stderr, 'Error: missing parameters.'
-            return 1
+            return 2
 
         for optim_name in optims.split():
             try:
@@ -211,8 +211,6 @@ class HaveSex(ReplApplication):
         if store:
             self.storage.save()
 
-        return 0
-
     def complete_optim(self, text, line, *ignored):
         args = line.split(' ')
         if len(args) == 2: