Gcov coverage plugin should throw an informative exception when report can't be parsed

[ChrisTimperley]

An example trace that shows the current behavior:

Traceback (most recent call last):
  File "/home/chris/.local/share/virtualenvs/darjeeling-ardupilot-5Zw1Ma5z/bin/darjeeling", line 33, in <module>
    sys.exit(load_entry_point('darjeeling', 'console_scripts', 'darjeeling')())
  File "/home/chris/tools/darjeeling/src/darjeeling/cli/__init__.py", line 309, in main
    app.run()
  File "/home/chris/.local/share/virtualenvs/darjeeling-ardupilot-5Zw1Ma5z/lib/python3.7/site-packages/cement/core/foundation.py", line 916, in run
    return_val = self.controller._dispatch()
  File "/home/chris/.local/share/virtualenvs/darjeeling-ardupilot-5Zw1Ma5z/lib/python3.7/site-packages/cement/ext/ext_argparse.py", line 808, in _dispatch
    return func()
  File "/home/chris/tools/darjeeling/src/darjeeling/cli/__init__.py", line 124, in coverage
    session = Session.from_config(environment, cfg)
  File "/home/chris/tools/darjeeling/src/darjeeling/session.py", line 86, in from_config
    coverage = cfg.coverage.build(environment, program)
  File "/home/chris/tools/darjeeling/src/darjeeling/coverage/config.py", line 104, in build
    coverage = collector.collect()
  File "/home/chris/tools/darjeeling/src/darjeeling/coverage/collector.py", line 71, in collect
    lines = self._extract(container)
  File "/home/chris/tools/darjeeling/src/darjeeling/coverage/gcov.py", line 189, in _extract
    return self._parse_xml_file_contents(xml_file_contents)
  File "/home/chris/tools/darjeeling/src/darjeeling/coverage/gcov.py", line 178, in _parse_xml_file_contents
    return self._parse_xml_report(root)
  File "/home/chris/tools/darjeeling/src/darjeeling/coverage/gcov.py", line 152, in _parse_xml_report
    assert packages_node
AssertionError