BUG: return value of "downloadExtension()" not always handled if 0
requested to merge github/fork/fbudin69500/downloadAndInstallExtensionMissingErrorMessage into master
The function downloadAndInstallExtension(const QString& extensionId) did not handled the case of downloadExtension() returning 0. We used the same approach as in scheduleExtensionForUpdate(const QString& extensionName) to handle that case correctly in downloadAndInstallExtension(const QString& extensionId).