index: raise error rather than crash on bad repo file

If a non-APK is added with the appid/packageName that matches some APKs, it
should through an error.

Traceback (most recent call last):
  File "/home/hans/code/fdroid/server/fdroid", line 22, in <module>
    fdroidserver.__main__.main()
  File "/home/hans/code/fdroid/server/fdroidserver/__main__.py", line 211, in main
    mod.main()
  File "/home/hans/code/fdroid/server/fdroidserver/update.py", line 2343, in main
    index.make(apps, sortedids, apks, repodirs[0], False)
  File "/home/hans/code/fdroid/server/fdroidserver/index.py", line 142, in make
    fdroid_signing_key_fingerprints)
  File "/home/hans/code/fdroid/server/fdroidserver/index.py", line 166, in make_v1
    v1_sort_packages(packages, fdroid_signing_key_fingerprints)
  File "/home/hans/code/fdroid/server/fdroidserver/index.py", line 292, in v1_sort_packages
    packages.sort(key=v1_sort_keys)
  File "/home/hans/code/fdroid/server/fdroidserver/index.py", line 288, in v1_sort_keys
    .format(apkfilename=package['apkName']))
fdroidserver.exception.FDroidException: at.roteskreuz.stopcorona_8.jobf does not have a valid signature!