python: Report detailed error message when opening fsread1 channel fails

Teach check_open() about validating the `message` part of `close`, when
given.

src/cockpit/channels/filesystem.py

@@ -109,8 +109,8 @@ class FsReadChannel(GeneratorChannel):
             return {'tag': '-'}
         except PermissionError:
             raise ChannelError('access-denied')
-        except OSError:
-            raise ChannelError('internal-error')
+        except OSError as error:
+            raise ChannelError('internal-error', message=str(error)) from error
 
 
 class FsReplaceChannel(Channel):

test/pytest/test_bridge.py

@@ -510,3 +510,9 @@ class TestBridge(unittest.IsolatedAsyncioTestCase):
         assert not all(d for d in data[0][0])
         # memory.used should be an integer
         assert isinstance(data[0][1], int)
+
+    async def test_fsread1_errors(self):
+        await self.start()
+        await self.transport.check_open('fsread1', path='/etc/shadow', problem='access-denied')
+        await self.transport.check_open('fsread1', path='/', problem='internal-error',
+                                        reply_keys={'message': "[Errno 21] Is a directory: '/'"})