handle exceptions that send a tuple for the error message

handle exceptions that send a tuple for the error message

Running `fdroid verify` I was seeing FDroidExceptions from verify.py:98
that had a tuple rather than a string.

Traceback (most recent call last):
  File "/home/hans/code/fdroid/server/fdroid", line 152, in <module>
    main()
  File "/home/hans/code/fdroid/server/fdroid", line 128, in main
    mod.main()
  File "/home/hans/code/fdroid/server/fdroidserver/verify.py", line 98, in main
    logging.info("...NOT verified - {0}".format(e))
  File "/home/hans/code/fdroid/server/fdroidserver/exception.py", line 22, in __str__
    ret += "\n==== detail begin ====\n%s\n==== detail end ====" % ''.join(self.detail).strip()
TypeError: sequence item 1: expected str instance, HTTPError found