docs: Improve documentation of Testplan.get_percentage

[rswarbrick]

Also, add two error conditions (for fractions that don't make sense).

[AlexJones0]

I'm pretty sure the round here is pointless - that's what the 2 in the format specifier is doing.
Python actually has a nifty format specifier for almost exactly this case:

Suggested change

	return "-- %" if total == 0 else f"{round(100 * value / total, 2):.2f} %"
	return "--%" if total == 0 else f"{value / total:.2%}"

But then you drop the space between the number and the % symbol. You could do:

Suggested change

	return "-- %" if total == 0 else f"{round(100 * value / total, 2):.2f} %"
	return "--%" if total == 0 else f"{value / total:.2%}".replace("%", " %")

Though that's about as much effort as the original.