Better exceptions

[ml31415]

At least the way I use the Betfair API so far, there are mainly three kinds of issues to distinguish in terms of how to respond to them:

• temporary issues, just wait and repeat
• needs relogin, connection loss or something
• login failed for permanent reasons, needs user intervention

The thrown errors should make it easy to distinguish between those categories with a simple except ... or isinstance(...) check. For that, the existing exception classes should be refactored.

Supersedes #29