Handle SSO login failure gracefully when unique email constraint is not met.

[shilob]

Occurs when multiple SSO methods uses the same email. For example, when AAF is/was enabled alongside the institution-specific SSO, and some users had logged in previously using one of the methods.

Logging in with an account that had used the same email will result in a silent request error, without notifying the user. Perhaps create a page to inform the user of the next steps?

@andrewbrazzatti what should be the next steps?