When Piwik cannot connect to the database, the API should return a valid response.

Reproduce:

• Edit config.ini.php and set a wrong username for the database eg. xyytueyeat
• Open index.php?module=API&method=API.getPiwikVersion&format=xml
• Got: HTML error output
• Expected:
  • HTTP error code 500,
  • and the API call returns a valid eg. XML response that returns the API error reponse eg.

<?xml version="1.0" encoding="utf-8" ?>
<result>
    <error message="Cannot connect to the database: SQLSTATE[28000] [1045] Access denied for user 'xyytueyeat'@'localhost' (using password: YES)" />
</result>

Steps:

• Add test case
• Fix issue

Refs #7901 #7902