provide a better ansible-inventory fallback error message old ansibles

see: https://github.com/ansible/awx/issues/3139

awx/main/management/commands/inventory_import.py

@@ -109,6 +109,17 @@ class AnsibleInventoryLoader(object):
         venv_exe = os.path.join(self.venv_path, 'bin', 'ansible-inventory')
         if os.path.exists(venv_exe):
             return venv_exe
+        elif os.path.exists(
+            os.path.join(self.venv_path, 'bin', 'ansible')
+        ):
+            # if bin/ansible exists but bin/ansible-inventory doesn't, it's
+            # probably a really old version of ansible that doesn't support
+            # ansible-inventory
+            raise RuntimeError(
+                "{} does not exist (please upgrade to ansible >= 2.4)".format(
+                    venv_exe
+                )
+            )
         return shutil.which('ansible-inventory')
 
     def get_base_args(self):