Add check for attempted nesting of .form-group

[cvrebert]

Refs twbs/bootstrap#17897

[tclindner]

Here are the docs I would like to add to the wiki.

Update home.md with

• [[E050]](.form-groups cannot be nested)

Add E050.md

E050

.form-groups should not be nested.

In Bootstrap v3, .form-groups do not support nesting.

Wrong:

<div class="form-group">
    …

    <div class="form-group">
        …
    </div>
</div>

Right:

<div class="form-group">
    …
</div>
<div class="form-group">
    …
</div>