Make errors in peakwidths! more friendly

[KronosTheLate]

I encountered one of the errors, and found that it could have been more helpful, and made the changes here.

Related note: would it not be better to add the missing field than to throw an error?

[halleysfifthinc]

I prefer not to automagically add the missing proms field, I think it is better to force the user to explicitly/consciously call the peakproms function, which will require them to intentionally accept the defaults or choose parameters which are tuned to the user's context/needs. This is also somewhat consistent with the old/explicit API. I

[KronosTheLate]

I had not though about the automagic solution leading to having to choose values for different parameters. That is a very good reason not to do too much. With the added hint, I also feel like there is not that much friction.

Thanks for the quick review!