Catch error on partner_users#create User already has the requested role so users don't get 500 error

[cielf]

Summary

If you try to set up a partner user that is already set up on that partner, it should give you a friendly, informative error, rather than a 500.

Why?

Reduction of bank user confusion

Details

To recreate:

sign in as [email protected]
Click Partners
select one of the partners
Manage Users
There will be at least one user already -- note the information for one of the users
Fill in the "Invite New User" information with that user's info, and click "Invite User"
You should see a nasty error page

What we want

We want to catch this and give a pleasant error "That user already has the requested role" will do.

Criteria for completion

• error appears as described
• automated tests to confirm