Improve and Tweak behaviour for --dropbox-refresh-token

[IAmVigneswaran]

@arjunprakash027

When --dropbox-refresh-token is uses, can we add a step / logic to clear the terminal screen before

INFO: Airlift version 1.0.6 appears?

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STEP: 1. Go to: https://www.dropbox.com/oauth2/authorize?response_type=code&client_id=.............
STEP: 2. Click "Allow" (you might have to log in first).
STEP: 3. Copy the authorization code.
Enter the authorization code here: 

Change the text from WARNING to STEP. That way the message would appear more user friendly.

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After success update of refresh token and when Refresh Token updated in the json file! is it possible to kill and close the terminal via code?

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Right now when user presses enter without authorization code, Airlift throws this error. We need to make it user friendly.

WARNING: error during retreival of refresh token
ERROR: Error at division
Traceback (most recent call last):
  File "airlift/dropbox_client.py", line 138, in change_refresh_access_token
  File "dropbox/oauth.py", line 350, in finish
  File "dropbox/oauth.py", line 189, in _finish
  File "requests/models.py", line 1021, in raise_for_status
requests.exceptions.HTTPError: 400 Client Error: Bad Request for url: https://api.dropboxapi.com/oauth2/token

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "airlift/cli.py", line 74, in cli
  File "airlift/dropbox_client.py", line 152, in change_refresh_access_token
airlift.utils_exceptions.CriticalError: error during retreival of refresh token

Maybe we can make it in this way.

1. WARNING: Error during retrieval of refresh token
2. Do you want to try again? (Y/N) IF Y, it would start the Loop with the App Key Again.
3. If N It would clear the terminal and close the terminal.