Include exception in logging.

[josephw]

The plot can fail for lots of reasons; rather than trying to predict them all, include the exception.

In my case, it was trying to write to a non-existent directory, rather than having a problem with missing data. It'd be possible to fix that, but just surfacing the underlying cause was enough to unblock me.

IMPORTANT: Please take note of the below checklist, especially the first two items.

Pull Request Checklist

• All pull requests must include the Contributor License Agreement (see below).
• Code should conform to the following:
  • pep8 compliant with some exceptions (see pytest.ini)
  • 100% test coverage with pytest (with valid tests). If you have difficulty
    writing tests for the code, feel free to ask for help or submit the PR without tests.
  • Complete, correctly-formatted documentation for all classes, functions and methods.
  • documentation has been rebuilt with tox -e docs
  • All modules should have (and use) module-level loggers.
  • Commit messages should be meaningful, and reference the Issue number
    if you're working on a GitHub issue (i.e. "issue #x - "). Please
    refrain from using the "fixes #x" notation unless you are sure that the
    the issue is fixed in that commit.
  • Git history is fully intact; please do not squash or rewrite history.

Contributor License Agreement

By submitting this work for inclusion in wifi-survey-heatmap, I agree to the following terms:

• The contribution included in this request (and any subsequent revisions or versions of it)
  is being made under the same license as the wifi-survey-heatmap project (the Affero GPL v3,
  or any subsequent version of that license if adopted by wifi-survey-heatmap).
• My contribution may perpetually be included in and distributed with wifi-survey-heatmap; submitting
  this pull request grants a perpetual, global, unlimited license for it to be used and distributed
  under the terms of wifi-survey-heatmap's license.
• I have the legal power and rights to agree to these terms.

[jantman]

Thanks so much!