update index before install to avoid errors when outdated index data point to non-existant releases

I got this issue reported by e-Mail for OpenTracks, but it is within the F-Droid client.

• Device OS and version: not relevant
• Device model/manufacturer: not relevant
• F-Droid version (in the About screen): unknown (just got a screenshot)
• F-Droid privileged extension version (if installed): no

Description

If a really old repository index is used, it might happen that the mentioned versions are not hosted anymore by F-Droid. If a user tries to install an application in this case, an error message is shown telling the user that the APK was not found (HTTP 404). This is not helpful as there is no call to action.

What did you do? (clear steps if possible)

Actual reason: I don't know why the repo index was not up to date.

Steps to reproduce:

1. Disable automatic repository fetching
2. Wait for quite some time (4 weeks) until more than 3 releases were triggered upstream and build by F-Droid.
3. Try to install

What did you expect to see?

A dialog that tells me that the repository index is quite old, probably outdated, and that I should update it (Dialog with Ok, Cancel).

What did you see instead?

A toast message telling me the URL of the APK and that it could not be found (aka HTTP 404, I guess).