Bootstrap - alert

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.min.css">
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.3/umd/popper.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/js/bootstrap.min.js"></script>
<div class="container">
    <h1>Login</h1>
    <div class="alert alert-warning" role="alert">
        <b>Aviso:</b> Senha incorreta
    </div>

    <form>
        <div class="form-group">
            <label for="exampleInputEmail1">Email</label>
            <input type="email" class="form-control" id="exampleInputEmail1" aria-describedby="emailHelp" placeholder="Enter email">
            <small id="emailHelp" class="form-text text-muted">Nunca compartilharemos seu email.</small>
        </div>
        <div class="form-group">
            <label for="exampleInputPassword1">Password</label>
            <input type="password" class="form-control" id="exampleInputPassword1" placeholder="Password">
        </div>
        <div class="form-check">
            <label class="form-check-label">
                <input type="checkbox" class="form-check-input"> Check me out
            </label>
        </div>
        <button type="submit" class="btn btn-primary">Submit</button>
    </form>
</div>

CSS

.container {
    background: #f0f0f0;
}