stylecheck: advise against non-conforming Unwrap, Is, and As methods on error types

[ainar-g]

(Inspired by golang/go#40356, so probably won't be as necessary if that proposal is accepted.)

What I did:

Filtered shell output

$ staticcheck --debug.version
staticcheck (devel, v0.0.1-2020.1.0.20200615045319-925e3c1d1b98)

Compiled with Go version: go1.14.6
Main module:
	honnef.co/go/[email protected] (sum: h1:8DVSlQMnJDu5uoPrBMZQcr2cz1aK/HdVSSIvhPzYx0c=)
Dependencies:
	github.com/BurntSushi/[email protected] (sum: h1:WXkYYl6Yr3qBf1K79EBnL4mak0OimBfB0XUf9Vl28OQ=)
	golang.org/x/[email protected] (sum: h1:KU7oHjnv3XNWfa5COkzUifxZmxp1TyI7ImMXqFxLwvQ=)
	golang.org/x/[email protected] (sum: h1:59syOWj4+Fl+op4LL8fX1kO7HmbdEWfxlw4tcGvH+y0=)
	golang.org/x/[email protected] (sum: h1:E7g+9GITq07hpfrRu66IVDexMakfv52eLZ2CXBWiKr4=)
$ nl -b a ./go/tmp.go 
     1	// +build ignore
     2	
     3	package main
     4	
     5	type myError struct{}
     6	
     7	func (myError) Error() (msg string)  { return "" }
     8	func (myError) Unwrap() (msg string) { return "" }
     9	
    10	func main() {
    11		var err error = myError{}
    12		_ = err
    13	}
$ staticcheck --checks=all -f stylish ./go/tmp.go
 ✖ 0 problems (0 errors, 0 warnings, 0 ignored)

What I expected: A warning about my error implementing the Unwrap interface incorrectly.

(This is stylecheck and not staticcheck, because this Unwrap could simply implement another interface for another library.)