improve error message for filter pass-by-value

[OscarVanL]

I was trying out the filter feature today on a struct, but made a mistake and did not pass the struct by reference/pointer:

type Foo struct {
	A string `filter:"trim"`
}

func main() {
	foo := Foo{A: "    test     "}

	v := validate.Struct(foo)
	if !v.Validate() {
		fmt.Println(v.Errors.String())
	}
}

It took me a little while to figure out the problem because the error message was not very clear:

_filter:
 _filter: set value failure

This MR improves the error message to be more informative. Hopefully it helps others debug easier :)

[coveralls]

Pull Request Test Coverage Report for Build 2936605061

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.17%

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Totals
Change from base Build 2928020510:	0.0%
Covered Lines:	2787
Relevant Lines:	2898

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💛 - Coveralls