auth: Report a more specific error with bad creds
auth: Report a more specific error with bad creds

Without this change, the resulting error message for an invalid password
is "authentication failed".

With this change, we report "Username and/or password are incorrect".

Fixes #4285

Signed-off-by: Jesse Haber-Kucharsky <526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>
Message-Id: <32d00be8af5075ee10d2c14f85b76843a9adac10.1551306914.git.526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>

avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla