`trim` and `guide` fun errors should use substituted fun names

[brodieG]

Low priority, may not be worth it, but it would be nice if for this type of error:

> diffPrint(letters, LETTERS, trim=function(x, y) stop("boom"))
Error in trim_fun(obj, obj.as.chr) : boom
Error in apply_trim(target, tar.capt.p, etc@trim) : 
  `*trim` method produced an error when attempting to trim ; If you did not define custom `*trim` methods contact maintainer (see `?trim`).

we actually got something more recognizable to the user in the first error, kind of like we do in unitizer.