new alert text from robinf. r=sspitzer #20401

mailnews/base/prefs/resources/locale/en-US/prefs.properties

@@ -27,7 +27,7 @@ enterAccountName=Please enter a name for this account.
 enterUserName=Please enter a username.
 enterName=Please enter your name.
 enterValidEmail=Please enter a valid email address.
-accountExists=An account of this type with the same username and hostname already exists.  Go back and change it or hit cancel.
+accountExists=A mail or newsgroup account with the same user name and server name already exists. Click Back and enter a different server name, or click Cancel.
 
 # when the wizard already has a domain (Should we say something different?)
 enterValidEmailPrefix=Please enter a valid email address.