src: improve logging messages

Some users have mistakenly reported tiptorrent problems when the process
takes a long time. Specifically by rebooting or powering off the client
in the middle of the md5sum computation stage, just after the tiptorrent
transfer.

Same problem occurs when image creation command takes a long period of
time.

In order to help the user understand the different stages of commands
such as image creation or image restore using tiptorrent, the following
changes have been made to the current logging solution:

- Add log messages to warn users not to reboot or shut down the client
  during a tiptorrent transfer, and also during the md5sum computation
  stage.

- Add a log message telling the user that the image creation processes
  have started.

- Use logging.exception inside "except:" blocks to print a traceback
  with the log messsage.
  (https://docs.python.org/3/library/logging.html#logging.exception)

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