Provide BaseType as string

[yanickrochon]

Right now, the implementation requires the actual base type prototype. Since errors are cached, it should be possible to pass either the base type as string, or the actual error type itself. If the base type, as string, is not a known error type, an exception should be thrown.

Therefore, instead of

var CustomError = errorFactory('CustomError', ..., errorFactory('BaseType'));

it should be

var CustomError = errorFactory('CustomError', ..., 'BaseType');