fix(get-poetry): use exit code 1 with non-existing version

Follow-up to python-poetry#3927, which
was reverted in python-poetry#3943,
now handling the "Latest version already installed." case
(python-poetry#3942).

get-poetry.py

@@ -352,7 +352,9 @@ def run(self):
         version, current_version = self.get_version()
 
         if version is None:
-            return 0
+            if current_version is not None:
+                return 0
+            return 1
 
         self.customize_install()
         self.display_pre_message()