Deal with zombie downloads

[paulmueller]

Sometimes, users are downloading data and then delete the target directory, going off to doing something else.

When a new download is started in DCOR-Aid, the download job is still there and the following error is displayed:

Dcor-aid
Unhandled exception in DCOR-Aid version 0.12.1:
Traceback (most recent call last):
  File "C:\Users\mkraete\AppData\Local\Programs\DCOR-Aid\dcoraid\gui\download\widget_download.py", line 131, in update_job_status
    status = job.get_status()
  File "dcoraid\download\job.py", line 250, in get_status
    if self.path.exists() and self.path.is_file():
  File "dcoraid\download\job.py", line 88, in path
    return self.get_download_path()
  File "dcoraid\download\job.py", line 140, in get_download_path
    raise ValueError(
ValueError: The `download_path` passed in __init__ is invalid. Please make sure the target directory for PATH_TO_FILE.rtdc exists.