Ruleset/basic validation: improve error messages

[jrfnl]

While the libxml error messages are far from perfect, they at least give some clue as to where to look for the error in a ruleset file.

This PR changes two things:

1. Previously, a PHP warning would also be thrown for a ruleset/installed standard which is not used in the current run.
   This warning is now silenced.
2. However, for the standards and rulesets actually used by the current run, any potential warnings are made more readable and will now also mention the file in which they were encountered.
   Previously the output would be along the lines of:

   Warning: simplexml_load_string(): Entity: line 82: parser error : Opening and ending tag mismatch: rule line 80 and rue in /path/to/PHP_CodeSniffer/src/Util/Standards.php on line 119
   Warning: simplexml_load_string():       </rue> in /path/to/PHP_CodeSniffer/src/Util/Standards.php on line 119
   Warning: simplexml_load_string():             ^ in /path/to/PHP_CodeSniffer/src/Util/Standards.php on line 119
   

   Now, it will look like:

   ERROR: Ruleset /path/to/ruleset.xml is not valid
   - On line 82, column 10: Opening and ending tag mismatch: rule line 80 and rue
   

Loosely related to #2188

[gsherwood]

I never got any of those XML errors - only the PHPCS exception. I must have a different config.

But these changes are really good because now I get errors, and they are nicely formatted. Thanks.