Forwarding to a running pardes says why in words: a name with a newline is "a file name is one line", never NotOneLine

The forwarding refusal printed the error's name for anything the
session itself did not answer: NotOneLine, NotAFileName,
NoWorkingDirectory. Those are now sentences, a dial failure is said as
the message row says one, and any other error is its name's words.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 8 insertions, 0 deletions

diff --git a/test/fs.py b/test/fs.py
index 9392ffb7..eba5a5fa 100644
--- a/test/fs.py
+++ b/test/fs.py
@@ -1391,6 +1391,14 @@ def test(binary, quic=False):
             assert refused.returncode == 1, refused
             assert b'bad character in file name' in refused.stderr, refused.stderr
             assert serials() == before, (serials(), before)
+            # A name with a newline is said in words, not an error's name.
+            for word, said in (('two\nlines.txt', b'a file name is one line'),):
+                refused = subprocess.run([binary, word], cwd=root, env=env,
+                                         stdout=subprocess.PIPE, stderr=subprocess.PIPE, timeout=10)
+                assert refused.returncode == 1, refused
+                assert said in refused.stderr, refused.stderr
+                assert b'NotOneLine' not in refused.stderr and b'NoSuchPane' not in refused.stderr
+                assert serials() == before, (serials(), before)
             orphan = launch('--wait')
             assert orphan.poll() is None
         assert orphan.wait(timeout=5) == 1