better config/CLI args validation

```
karma start test/e2e/basic/karma.conf.js --log-level

/Users/vojta/Code/karma/node_modules/log4js/lib/logger.js:56
return this.level.isLessThanOrEqualTo(otherLevel);
^
TypeError: Cannot call method 'isLessThanOrEqualTo' of undefined
at Logger.isLevelEnabled (/Users/vojta/Code/karma/node_modules/log4js/lib/logger.js:56:23)
at Logger.(anonymous function) [as debug] (/Users/vojta/Code/karma/node_modules/log4js/lib/logger.js:67:22)
at requirePlugin (/Users/vojta/Code/karma/lib/plugin.js:12:9)
at /Users/vojta/Code/karma/lib/plugin.js:38:9
at Array.forEach (native)
at Object.exports.resolve (/Users/vojta/Code/karma/lib/plugin.js:25:11)
at Object.exports.start (/Users/vojta/Code/karma/lib/server.js:254:35)
at Object. (/Users/vojta/Code/karma/bin/karma:20:39)
at Module._compile (module.js:456:26)
at Object.Module._extensions..js (module.js:474:10)
at Module.load (module.js:356:32)
at Function.Module._load (module.js:312:12)
at Function.Module.runMain (module.js:497:10)
at startup (node.js:119:16)
at node.js:901:3
```