Show warning message when trying to create SU with existing name

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 d2bcce2b1c3355bbb5bc305a9a35c25de2c80602..bcfe85c22df8dfb7ccd58feb589e7cece816f876 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -12930,6 +12930,9 @@ Some features may be unavailable.  Please check that the profile exists and you
         <message name="IDS_PROFILES_CREATE_EXISTING_LEGACY_SUPERVISED_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;a is="action-link" id="supervised-user-import-existing"&gt;</ph>import <ph name="PROFILE_NAME">$1<ex>John</ex></ph> to this device<ph name="END_LINK">&lt;/a&gt;</ph>?
         </message>
+        <message name="IDS_PROFILES_CREATE_EXISTING_LEGACY_SUPERVISED_USER_ERROR_EXISTS_LOCALLY" desc="Message shown when the user enters the name of a supervised user that we already has locally.">

[Marc Treib, 2015/12/10 11:36:41]

nit: That we already haVE

[atanasova, 2015/12/15 17:57:36]

Done.

+          Looks like you have already imported a supervised user with this name.

[Marc Treib, 2015/12/10 11:36:41]

Hm, we haven't necessarily imported it, we might just have created it here.
Maybe just "Looks like you already have a supervised user with this name."?

Also, I just noticed that on ChromeOS we already have a string for this case:
IDS_CREATE_SUPERVISED_USER_CREATE_USERNAME_ALREADY_EXISTS
"Whoops, this name is already in use!"
Maybe we should reuse that one here, or maybe not, since the "style" is somewhat
different... let's see what Pam thinks.

[Pam (message me for reviews), 2015/12/15 14:31:36]

"Looks like you already have a supervised user with this name." sounds good, at
least once the functionality is changed to disallow all dupes rather than only
local ones. Otherwise we probably want to add "...on this device" or similar.

[atanasova, 2015/12/15 17:57:36]

Changed it to Pam's suggestion without the "on this device". I think that since
users with duplicate names could have been created, we cannot assume if all of
them are on the device or not.

+        </message>
         <message name="IDS_PROFILES_CREATE_SUPERVISED_SIGNED_IN_LABEL" desc="Label for the 'Supervised user' checkbox in the create-profile dialog when the current user is signed in.">
           Control and view the websites this person visits from <ph name="CUSTODIAN_EMAIL">$1<ex>user@gmail.com</ex></ph>.
         </message>