throw error on missing partial

[jakubgs]

Fixes: #3384

[coveralls]

Coverage decreased (-0.001%) to 97.261% when pulling c398b44 on jakubgs:master into a027779 on hexojs:master.

[tcrowe]

@jakubgs It's good and clean

@yoshinorin You said it's better to throw than log a message here. I'm not sure so I will defer to your judgement.

[yoshinorin]

@tcrowe
It is possibility that user can not notice about broken layout, if only a log message. I think an exception is better than log message, in order to notice it to user.

I'm going to test it by my local machine. But I have been busy recently with daily routine. 😓 So, please wait patiently. Thanks :)

[tcrowe]

@yoshinorin No problem! If you want to I can do it. You may hand over any task to me or assign it to me until you are ready to come back.

[tomap]

There is an eslint error before validating

[tcrowe]

The tests fail here:
https://github.com/hexojs/hexo/blob/master/test/scripts/helpers/partial.js#L46

[jakubgs]

Now it seems the only blocker is the ridiculously small reduction in coverage. No idea what to do about that.

[yoshinorin]

LGTM :)
@jakubgs Thanks.