Crash when trying to load binary incompatible data file

Trying to load a binary incompatible data file produces a crash.
(This is probably an other reason to switch to JSON.)

* Broken data files must produce a helpful error message.
* Also improve error messages for broken configuration files. The long message is almost never displayed.

Cause: The library package for `bsync` was renamed, and these names seem to be imbedded into the data files.

Back trace:

Traceback (most recent call last):
  File "/usr/local/bin/bsync", line 31, in <module>
    bsync.main()
  File "/usr/local/lib/python2.7/site-packages/bug_repo_syncer/main.py", line 116, in main
    self.do_info()
  File "/usr/local/lib/python2.7/site-packages/bug_repo_syncer/main.py", line 295, in do_info
    self.read_data()
  File "/usr/local/lib/python2.7/site-packages/bug_repo_syncer/main.py", line 436, in read_data
    self.known_bugs = unp.load()
`ImportError`: No module named bug_syncer.common