Re-run add-on installation when failing on a package

[janezd]

Moved from biolab/orange3#2407.

@ales-erjavec (or another person who knows this code), please check whether the fix would be too complicated; if so, just close the issue.

Expected behavior

Add-ons that can be installed, get installed on Orange.

Actual behavior

When one add-on installation fails, the add-on window closes and none of the checked add-ons after the failing one gets installed.

Steps to reproduce the behavior

Windows. Try to install Networks or Timeseries. Then tick an add-on that is listed after one of these two. The add-ons fail and the final add-on doesn't get installed because the windows closes.

Additional info (worksheets, data, screenshots, ...)

Can we make the add-on installation continue after the error was reported? Perhaps catch the error, write it as the status of the add on (e.g. Timeseries - Error), then run the installation for the rest of the selected add-ons?