_frontend/widget.py: Change how we format failure messages.

  o Now print the message itself on the same line in place of the abbreviated log file

  o When reading back the last lines of the log file, prefix that
    with an informative message about whence the log lines come,
    showing the full unabbreviated log file.

  o Fixed the behavior of read_last_lines() so that it works as
    intended, problems were:

    - Was prepending an additional newline when the log contains
      more lines than we want to print
    - Was not printing anything at all in the case we want to
      print more lines than exist in the log