CAS-1244 Restore error handling behavior.

Commit baf4ff7f7d9f0c60341be70aad49b5895667da20 broke the documented behavior of AuthenticationManagerImpl and the undocumented behavior of the other AuthetnicationManager implementations. The former behavior, to throw the first exception of any kind produced by an AuthenticationHandler, has been restored.
CAS-1244 Refactor error handling to reduce code duplication.
Merge pull request #189 from serac/cas-1244

CAS-1244 Restore error handling behavior of AuthenticationManager components.