refactor(lib): use assert library instead of throwing error directly

[lusarz]

NodeJS includes assertion library natively, and it's syntax seems to be more elegant that throwing errors directly.

[johnjbarton]

Does this impact the user experience? The message is similar?

[lusarz]

I see message like:

previously it was:

I think we can go one step further - wrap assert and create something like:

assertArrayWhenDefined(config.browsers, 'Invalid configuration: browsers option must be an array')

etc.

@johnjbarton What do you think ?

[johnjbarton]

I think your change is good as is.

[lusarz]

Ok.
Maybe a bit offtopic - seems like we have two different ways of processing fatal errors:

• here we throw error out:
  

  karma/lib/config.js

  Line 200 in 385a0f3

  assert(Array.isArray(config.browsers), 'Invalid configuration: browsers option must be an array')

• here we log error and do process.exit :
  

  karma/lib/config.js

  Lines 381 to 382 in 385a0f3

  	log.error('Config file must export a function!\n' + CONFIG_SYNTAX_HELP)
  	return process.exit(1)

Would be nice to make it more consistent one day :)