Multi-LDAP: Do not fail-fast on invalid credentials (#19261)

* Multi-LDAP: Do not fail-fast on invalid credentials

When configuring LDAP authentication, it is very common to have multiple
servers configured. When using user bind (authenticating with LDAP using
the same credentials as the user authenticating to Grafana) we don't
expect all the users to be on all LDAP servers.

Because of this use-case, we should not fail-fast when authenticating on
multiple LDAP server configurations. Instead, we should continue to try
the credentials with the next LDAP server configured.

Fixes #19066

pkg/services/multildap/multildap_test.go

@@ -152,6 +152,25 @@ func TestMultiLDAP(t *testing.T) {
 				teardown()
 			})
 
+			Convey("Should still try to auth with the second server after receiving an invalid credentials error from the first", func() {
+				mock := setup()
+
+				mock.loginErrReturn = ErrInvalidCredentials
+
+				multi := New([]*ldap.ServerConfig{
+					{}, {},
+				})
+				_, err := multi.Login(&models.LoginUserQuery{})
+
+				So(mock.dialCalledTimes, ShouldEqual, 2)
+				So(mock.loginCalledTimes, ShouldEqual, 2)
+				So(mock.closeCalledTimes, ShouldEqual, 2)
+
+				So(err, ShouldEqual, ErrInvalidCredentials)
+
+				teardown()
+			})
+
 			Convey("Should return unknown error", func() {
 				mock := setup()