build: check that metadata is present before creating tmp dirs
build: check that metadata is present before creating tmp dirs

Before, `fdroid build` would create some subdirs, then fail when it could
not read the metadata via metadata.read_metadata().  This checks before
whether there is any metadata available, and warns the user accordingly.

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