sattelite prediction without filename crashes

[ReimarBauer]

CRITICAL: Fatal error: Traceback (most recent call last):
  File "/home/reimar/PycharmProjects/MSS/mslib/msui/satellite_dockwidget.py", line 155, in load_file
    overpass_segments = read_nasa_satellite_prediction(filename)
  File "/home/reimar/PycharmProjects/MSS/mslib/msui/satellite_dockwidget.py", line 64, in read_nasa_satellite_prediction
    satfile = _fs.open(_name, 'r')
  File "/home/reimar/miniconda3/envs/mssdev/lib/python3.9/site-packages/fs/osfs.py", line 642, in open
    raise FileExpected(path)
fs.errors.FileExpected: path '' should be a file

[ReimarBauer]

"load" button may be only active when a file was selected

and also a failure message could be shown as popup on the exception