support reraise of custom exceptions within core exception handling

[Mike Bayer]

patch attached.

[Mike Bayer]

• The mechanics of the :meth:.ConnectionEvents.dbapi_error handler have been enhanced such that the function handler is now capable of raising or returning a new exception object, which will replace the exception normally being thrown by SQLAlchemy. fixes #3076

→ <<cset e0a9b94abb92>>

[Mike Bayer]

• The mechanics of the :meth:.ConnectionEvents.dbapi_error handler have been enhanced such that the function handler is now capable of raising or returning a new exception object, which will replace the exception normally being thrown by SQLAlchemy. fixes #3076

→ <<cset f0c318b069c1>>

[Mike Bayer]

we need a lot more functionality here so we're adding a new event, leaving dbapi_error alone.

[Mike Bayer]

946b92fcf9dabe5014236b82 dfb5707dae454448ab3e34e9c4ff