text: Title: $XONSH_TRACEBACK_LOGFILE with '~' crashes xonsh
  Body: ## Steps to Reproduce
  
  ```sh
  $XONSH_TRACEBACK_LOGFILE = "~/log"
  raise Exception
  ```
  
  ## xonfig
  
  Current master.
  
  ## Expected Behavior
  Log gets written without errors.
  
  ## Current Behavior
  
  Xonsh crashes [here](https://github.com/xonsh/xonsh/blob/8e1a1f3342bc30e9b5c0d58e4713daf8e23295d0/xonsh/tools.py#L976) with `FileNotFoundError: [Errno 2] No such file or directory: '<current path>/~/log'`.
  
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