Tests to verify part one of #1617 - you can't start changing your email address if there is already a conflicting email address.
Change the confirm email action to present user friendly error messages instead of error pages. Update fact coverage of TheConfirmAction.
Make UserService refuse (throw) to confirm email addresses which would create duplicate confirmed addresses.
Show an error when you try to confirm an email address thats already confirmed.
Last part of fixing #1613. For users who got duplicated email addresses they will now see that their account can't be found instead of seeing a white screen of death.
Bugfixes from rerunning all tests.
Merge pull request #1619 from NuGet/tilovell/1613-BetterHandlingOfDuplicateEmailAddresses

Tilovell/1613 better handling of duplicate email addresses
Merge branch 'qa-next' into dev-start

Conflicts:
	src/NuGetGallery/Services/UserService.cs
	src/NuGetGallery/Strings.Designer.cs
	src/NuGetGallery/Strings.resx
	tests/NuGetGallery.Facts/Controllers/UsersControllerFacts.cs
Fixed some merge conflicts