build: better logging output on rsync failures
build: better logging output on rsync failures

Save rsync error output and combine that with the command invocation
into an FDroidException which can be logged to the wiki.

This additionally sets -q for rsync to only print errors.

f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server