Add 'Error' screen reader text to file integrity table

[audrasjb]

Short introduction

• Related: "Error" cell contains an empty span element in file integrity table #267

Description of what the PR accomplishes

Proposed HTML markup:

<tr>
   <td><span class="screen-reader-text">Error</span><span class="error"></span></td> 
   <td>/wp-admin/js/widgets.js</td><td>Content changed <a href="#health-check-diff" data-file="wp-admin/js/widgets.js">(View Diff)</a></td>
</tr>

PR Checklist

These are things to ensure are covered by your PR.

• This PR is created against the develop branch
• This PR is feature ready and can be reviewed in its entirety

[Clorith]

I took your base here, and expanded on it in #278 as there were other locations also needing the same treatment 👍