Simplify error handling in account username setting

Move the check for valid account pattern down into AccountSecurityImpl
and raise InvalidUserNameException from there.

Remove the invalidUserName utility method which is no longer needed
as we now only need to generate the error dialog at one place.

Check for the exception being an instance of InvalidUserNameException
rather than doing a string comparison of the message.

Change-Id: Ia4a9df88318f8459e8dc4e63368d896791dd9a18

gerrit-httpd/src/main/java/com/google/gerrit/httpd/rpc/account/AccountSecurityImpl.java

@@ -20,6 +20,7 @@ import com.google.gerrit.common.ChangeHooks;
 import com.google.gerrit.common.data.AccountSecurity;
 import com.google.gerrit.common.data.ContributorAgreement;
 import com.google.gerrit.common.errors.ContactInformationStoreException;
+import com.google.gerrit.common.errors.InvalidUserNameException;
 import com.google.gerrit.common.errors.NoSuchEntityException;
 import com.google.gerrit.common.errors.PermissionDeniedException;
 import com.google.gerrit.httpd.rpc.BaseServiceImplementation;
@@ -109,10 +110,13 @@ class AccountSecurityImpl extends BaseServiceImplementation implements
   public void changeUserName(final String newName,
       final AsyncCallback<VoidResult> callback) {
     if (realm.allowsEdit(Account.FieldName.USER_NAME)) {
+      if (newName == null || !newName.matches(Account.USER_NAME_PATTERN)) {
+        callback.onFailure(new InvalidUserNameException());
+      }
       Handler.wrap(changeUserNameFactory.create(newName)).to(callback);
     } else {
-      callback.onFailure(new PermissionDeniedException("Not allowed to change"
-          + " username"));
+      callback.onFailure(
+          new PermissionDeniedException("Not allowed to change username"));
     }
   }