Prompt the user to create an account if they try to log in with an unknown email #2870754

When testing Jetpack’s signup flow, I felt it was awkward if I tried to log in with a previously unknown email:

• the link to create an account is not very prominent, and possibly below the fold on mobile devices
• the error doesn’t contain an action the user can take, e.g. creating a new account

Changes proposed in this Pull Request

• Prompt user to create an account if they try to log in with an unknown email

Screenshots of this PR in action

… then after clicking the link…

Testing instructions

• Connect a Jetpack site
• Attempt to log in using a previously-unregistered email
• You should be prompted to “create a new account” in the error message
• Clicking to create an account should take you to the correct screen to do so