Add more info regarding trailing slash during prefix creation

Some Ozone users often forget to add the trailing slash in prefix ACL operations (i.e. ozone sh prefix setacl, ...), causing confusing PREFIX_NOT_FOUND error. 

This patch should add more info about trailing slash in exception message.

You can refer to HDDS-7933 for more documentation on the prefix ACL path convention.

Hint: See PrefixManagerImpl#validateOzoneObj