login fails with oops if email assigned to another user

# Background

I have an existing lp account. A few days ago I created a new account, specifically for work-related activities. When I created this account I specified my work email address. Launchpad successfully created this second account.

# Launchpad oops

Attempting to login to my new lp account failed consistently with an Oops like this:

(Error ID: OOPS-60009aecf89aad62087222cb55edd62a}

# User error! ;)

Talking to @jugmac00 on #launchpad showed the reason for the oops: I'd forgotten that I'd added my work email address to my existing (personal) lp account as a secondary email address.

# Solution

The fix was simple: remove my work email address from my personal lp account. lp then allowed me to login using my new lp account which specified my work email address.

## Possible lp bugs

- Launchpad should arguably not have allowed me to create a new account if an existing account was linked to the email address I specified for the new account.

- Launchpad should have given a friendly human-readable error message such as, "ERROR: Your primary email address is linked to another account ($the_other_account)" rather than a scary looking oops message.