go/analysis/passes/printf: adjust error message

Rather than stating a neutral-sounding fact
(i.e., "Sprintf call has error-wrapping directive %w"),
make it more clear why this is wrong
(e.g., "Sprintf call has error-wrapping directive %w, which only operates with Errorf")

Change-Id: I3417423bcc09e6c9ede9993c33d8420ec57c2a08
Reviewed-on: https://go-review.googlesource.com/c/tools/+/317431
Trust: Joe Tsai <joetsai@digital-static.net>
Trust: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

go/analysis/passes/printf/printf.go

@@ -590,7 +590,7 @@ func checkPrintf(pass *analysis.Pass, kind Kind, call *ast.CallExpr, fn *types.F
 		}
 		if state.verb == 'w' {
 			if kind != KindErrorf {
-				pass.Reportf(call.Pos(), "%s call has error-wrapping directive %%w", state.name)
+				pass.Reportf(call.Pos(), "%s call has error-wrapping directive %%w, which is only supported by Errorf", state.name)
 				return
 			}
 			if anyW {