init: fix test for aapt when no aapt has been found

init: fix test for aapt when no aapt has been found

Just give a proper error message rather than this stack trace:

Traceback (most recent call last):
  File "/home/hans/code/fdroid/server/fdroid", line 156, in <module>
    main()
  File "/home/hans/code/fdroid/server/fdroid", line 132, in main
    mod.main()
  File "/export/share/code/fdroid/server/fdroidserver/init.py", line 148, in main
    if os.path.isfile(aapt):
  File "/usr/lib/python3.5/genericpath.py", line 30, in isfile
    st = os.stat(path)