Creating a user that already exists does not show a helpful message, it just says that it failed:

CREATE USER 'foo'@'bar.com';

CREATE USER 'foo'@'bar.com';

ERROR 1396 (HY000): Operation CREATE USER failed for 'foo'@'bar.com'

This is not helpful at all and is not in line with other CREATE statements, such as create table:

CREATE TABLE foo(bar INT);

CREATE TABLE foo(bar INT);

ERROR 1050 (42S01): Table 'foo' already exists

An error message is supposed to tell you what went wrong, if possible, not only that it went wrong, just like with the CREATE TABLE statement above, and not as in the CREATE USER statement. This is also true for RENAME USER.