[MERGE][0.15] Better error on incompatible formats

[John Arbash Meinel]

John Arbash Meinel has voted +1.
Status is now: Approved
Comment:
I would be more comfortable with:

inter = InterRepository.get()
try:
   inter.fetch()
except NotImplementedError:
   raise Incompatibile...

So that we don't accidentally suppress something in the get() call.

I also think this would be reasonable for 0.15, since it is a very 
localized change. But that is up to Martin. (And maybe it should have a 
NEWS entry either way)

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C460A775B.2050804%40utoronto.ca%3E