Add fallback factory

[RezMike]

Now you can only specify a fallback value:

ExceptionMappersStorage.setFallbackValue(MR.strings.common_unknownError.desc())

Another option is to add a fallback factory:

ExceptionMappersStorage.setFallbackFactory { error : Throwable ->
    if (error.message.isNullOrBlank()) {
        MR.strings.common_unknownError.desc()
    } else {
        error.message!!.desc()
    }
}

[Alex009]

will be released in 0.7.0