handle StopIteration properly

This commit addresses the bug described
in:
<datalad#596 (comment)>.

The reason for the `AttributeError` was that
un-caught `StopIteration` exceptions prevented
an initialization of the `returncode`-attribute.
This is fixed by initializing the attribute
in the constructor of
`ShellCommandResponseGenerator`.

The commit adds a regression test that ensures
that the correct error is signaled.

Author: christian-monch

datalad_next/utils/shell_connection.py

@@ -74,7 +74,8 @@ def train(queue: Queue):
             if result_0.returncode != 1:
                 raise RuntimeError(
                     'unexpected return code from command'
-                    f'`test`: {result_0.returncode}')
+                    f'`test`: {result_0.returncode}, '
+                    f'stderr: {b"".join(result_0.stderr_deque)}')
 
             # Return the now ready connection
             yield cmd_executor
@@ -271,7 +272,7 @@ def __init__(self,
         self.end_marker = end_marker
         self.state = 'output'
         self.returncode_chunk = b''
-        self.returncode: int
+        self.returncode: int | None = None
 
     def send(self, _):
         if self.state == 'output':

datalad_next/utils/tests/test_shell_connection.py

@@ -222,3 +222,10 @@ def test_delete(sshserver):
         # verify that the remote files were deleted
         for file in files_to_delete:
             assert not (local_path / file).exists()
+
+
+def test_returncode():
+    with pytest.raises(RuntimeError) as e:
+        with shell_connection(['ssh', 'xyz@localhost:22']) as ssh:
+            pass
+    print(repr(e))