Allow completely changing error output prefix.

e.g., I want Error: to all be red & underlined when connected to a terminal; I don't want the colon to be normal color, non-underlined text.

ParsableArguments#_errorLabel is always output with a : suffix, so I cannot change the colon's color or underline it.

Add ParsableArguments#_errorPrefix that defaults to `"(_errorLabel): ", so ": " can be overridden.