Rename the format function for matchers

[minkimcello]

From a discussion in PR #902:

is format only used to format the error message? if so can we name it more explicitly like formatErrorMessage? format reads a bit confusing imo because it is also makes me think that I'll use format to check a value complies with a format or something like that

I think we could call it error() or errorMessage().

[cowboyd]

I think we could call it error() or errorMessage().

It isn't just an error message, it's also used in locators:

HTML(including("Hello World")).click():

couldn't find html element including "Hello World"
did you mean one of:
Hello Mom
Hello Planet
Hello Universe

In that case, is the string including "Hello World" really an error message even though it appears inside an error message?

Also, in our inspector These could appear matchers that you are just evaluating to play around, and we'd want to render them html including "Hello World" and there would be no error message at all.

[minkimcello]

@cowboyd Oh I see. Then maybe description() as you suggested would be most appropriate