ssh: Fix folder pull on SSH connection

Paramiko seems to have had a slight change in behavior that broke
devlib: to save a remote command execution, we attempt to pull any path
as file first, and then as a folder if the former failed.

This is now broken as paramiko will create an empty destination file
when trying to pull as a file. When we attempt again to pull as folder,
the destination exists already (empty file) and we raise an exception.

To fix that, make sure we cleanup any attempt after pulling as a file
before trying again.

devlib/utils/ssh.py

@@ -499,7 +499,18 @@ class SshConnection(SshConnectionBase):
         push(sftp, src, dst, callback)
 
     def _pull_file(self, sftp, src, dst, callback):
-        sftp.get(src, dst, callback=callback)
+        try:
+            sftp.get(src, dst, callback=callback)
+        except Exception as e:
+            # A file may have been created by Paramiko, but we want to clean
+            # that up, particularly if we tried to pull a folder and failed,
+            # otherwise this will make subsequent attempts at pulling the
+            # folder fail since the destination will exist.
+            try:
+                os.remove(dst)
+            except Exception:
+                pass
+            raise e
 
     def _pull_folder(self, sftp, src, dst, callback):
         os.makedirs(dst)