Add error handling for supervised user import flow.

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index 52620b1d1dea74eb06900ec17254d42452ca3d13..ba55788d557b2b32a1ffde2271281850bb6c3062 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -14613,6 +14613,9 @@ Some features may be unavailable.  Please check that the profile exists and you
     <message name="IDS_PROFILES_CREATE_SIGN_IN_ERROR" desc="Message shown when a sign-in error occurs during creation of a new supervised user.">
       Oops! The new supervised user couldn't be created. Please make sure you're signed in properly and try again.
     </message>
+    <message name="IDS_PROFILES_CREATE_EXISTING_MANAGED_USER_ERROR" desc="Message shown when the user enters the name of a supervised user that can be imported.">
+    Looks like you're already managing a user by that name.<ph name="LINE_BREAK">&lt;br/&gt;</ph>Did you want to <ph name="BEGIN_LINK">&lt;button id="supervised-user-import" class="link-button"&gt;</ph>import <ph name="PROFILE_NAME">$1<ex>John</ex></ph> to this device<ph name="END_LINK">&lt;/button&gt;</ph>?
+  </message>
     <message name="IDS_PROFILES_CREATE_MANAGED_SIGNED_IN_LABEL" desc="Label for the 'Supervised user' checkbox in the create-profile dialog when the current user is signed in. This will be followed by a 'Learn more' link.">
       This is a supervised user managed by <ph name="CUSTODIAN_EMAIL">$1<ex>user@gmail.com</ex></ph>.
     </message>