rgw/radosgw-admin clarify error when email address already in use

rgw/radosgw-admin clarify error when email address already in use

The error message if you try and create an S3 user with an email
address that is already associated with another S3 account is very
confusing; this patch makes it much clearer

To reproduce:

radosgw-admin user create --uid=foo --display-name="Foo test" --email=bar@domain.invalid
radosgw-admin user create --uid=test --display-name="AN test" --email=bar@domain.invalid
could not create user: unable to parse parameters, user id mismatch, operation id: foo does not match: test

With this patch:

radosgw-admin user create --uid=test --display-name="AN test" --email=bar@domain.invalid
could not create user: unable to create user test because user id foo already exists with email bar@domain.invalid

Fixes: https://tracker.ceph.com/issues/49137
Fixes: https://tracker.ceph.com/issues/19411
Signed-off-by: Matthew Vernon <mv3@sanger.ac.uk>
(cherry picked from commit 05318d6f71e45a42a46518a0ef17047dfab83990)