Config and HttpDataCollector improvements

[gregurco]

• [ ] Bug fix
• [x] New feature
• [ ] BC breaks
• [ ] Deprecations
• [x] Tests pass

Fixed tickets: [] License: MIT

What I did:

• Validate that form_params and multipart can't be used at the same time
• HttpDataCollector::getErrorCount method (there was TODO)
• Improve annotation for logger
• Tests for invalid options and HttpDataCollector::getErrorCount

[florianpreusner]

Very nice! Thanks a lot, @gregurco!