Me: improve error message when changing email to a currently used email address

[v18]

There are two related bugs in the changing email process:

1. The error message does not give enough information
2. The unusable email the user entered remains on the page

Steps to reproduce:

(1) Go to me/account

(2) Enter an email that is already used for a different WordPress.com account in the 'email address' field

(3) Click 'Save Account Settings'

(4) See that the email entered in step 2 remains in the email address field, even though it cannot be used. See that the error message does not tell you that you cannot use the same email twice.

Instead here's what should happen:

(1) The user should see an error message like:

'The email address [email protected] is already being used for another WordPress.com account. Please use a different email'

(2) The email field should show the currently used email address for the account.

[rickybanister]

It should be contextual and look like this:

(from the pattern library)

[ebinnion]

We should improve error messaging and field validation throughout /me. So, closing this issue in favor of #1647.