Give descriptive error if auth method not found (#10163)

* Give descriptive error if auth method not found

Previously during a `consul login -method=blah`, if the auth method was not found, the
error returned would be "ACL not found". This is potentially confusing
because there may be many different ACLs involved in a login: the ACL of
the Consul client, perhaps the binding rule or the auth method.

Now the error will be "auth method blah not found", which is much easier
to debug.

.changelog/10163.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+acl: Give more descriptive error if auth method not found.
+```

agent/consul/acl_endpoint.go

@@ -2380,7 +2380,7 @@ func (a *ACL) Login(args *structs.ACLLoginRequest, reply *structs.ACLToken) erro
 	if err != nil {
 		return err
 	} else if method == nil {
-		return acl.ErrNotFound
+		return fmt.Errorf("%w: auth method %q not found", acl.ErrNotFound, auth.AuthMethod)
 	}
 
 	if err := a.enterpriseAuthMethodTypeValidation(method.Type); err != nil {

agent/consul/acl_endpoint_test.go

@@ -4628,7 +4628,7 @@ func TestACLEndpoint_Login(t *testing.T) {
 		}
 		resp := structs.ACLToken{}
 
-		testutil.RequireErrorContains(t, acl.Login(&req, &resp), "ACL not found")
+		testutil.RequireErrorContains(t, acl.Login(&req, &resp), fmt.Sprintf("auth method %q not found", method.Name+"-notexist"))
 	})
 
 	t.Run("invalid method token", func(t *testing.T) {

command/login/login_test.go

@@ -143,7 +143,7 @@ func TestLoginCommand(t *testing.T) {
 
 		code := cmd.Run(args)
 		require.Equal(t, code, 1, "err: %s", ui.ErrorWriter.String())
-		require.Contains(t, ui.ErrorWriter.String(), "403 (ACL not found)")
+		require.Contains(t, ui.ErrorWriter.String(), "403 (ACL not found: auth method \"test\" not found")
 	})
 
 	testSessionID := testauth.StartSession()