Retain me value through error flow

[fluffy-critter]

If there's an error logging in, the disposition.Error should carry the failed identity URL so that the login form can present it as a default value.

Also, on the Flask handler, the error disposition could be passed through to the template, instead of using the message flashing mechanism which is a bit unwieldy.

[fluffy-critter]

Unfortunately this isn't possible to stash into the Disposition, as most handlers only actually have the identity URL if login has either succeeded (in which case there's no point in floating up an error) or has failed detection (in which case the handler wouldn't have been selected to begin with). The only exceptions are silo providers like Twitter and Facebook and those are kind of anathema to the point as well.

However, in the case of Flask it could be stashed into the session, and the Flask handler should use the error disposition instead of message flashing.