Download failure deletes satellites

[Paul Gilmartin]

It appears that if, while attempting to update satellites.json,
Stellarium experiences a momentary communication failure
to one of the source URLs, all satellites belonging to that URL
are deleted from satellites.json. Of course, they come back
at the next update when communication succeeds. But
metadata concerning groups, visibility, and radio frequencies
can be lost.

I can reproduce this behavior by temporarily changing one of
the sources to an invalid URL.

I have a couple circumventions:

o I have backed up the original satellites.json. If a problem
  occurs, I restore it and run update again from the plugins
  dialog.

o For other reasons (discussed in Bug #1260539 and
  Question #240712) I am dowloading the TLE files with
  a script using "cURL" and caching them. Only when
  my cache is intact I update satellites.json using
  file:// URLs (Thanks, Alexander, for that suggestion.)

Ideally, when communication to a source fails, the matching
modules/Satellites/*.tle should be left intact, not deleted,
and a message logged and displayed that the satellites.json
data base is (partially) obsolete.

Thanks,
gil