No destinction between exceptions in (Global)EntityCollection

[guest]

The EntityCollection in 0.7.0 (and GlobalEntityCollection in trunk) may raise a variety of exceptions in the "resolve" method:

http://elixir.ematia.de/trac/browser/elixir/trunk/elixir/collection.py#L69

When writing code that does black magic with entity collections (for example, arranging different entity collections in a stack and going through that stack to resolve a key) it is beneficial to differentiate between different exceptions that "resolve" may raise. In particular, "not found" and "found more that one" exceptions are two completely different outcomes and require completely different actions.

[ged]

This is a reasonable suggestion. Would you care enough to write a patch?