Improve error message for dupe srcs in genrule

The previous error message did not really give a good indication
of what the problems with my genrule was.

Test: Make a broken genrule
Change-Id: I4bb67f2792ca5a910f70354f10d0f81f330b5c96

genrule/genrule.go

@@ -290,7 +290,7 @@ func (g *Module) GenerateAndroidBuildActions(ctx android.ModuleContext) {
 		if _, exists := locationLabels[label]; !exists {
 			locationLabels[label] = loc
 		} else {
-			ctx.ModuleErrorf("multiple labels for %q, %q and %q",
+			ctx.ModuleErrorf("multiple locations for label %q: %q and %q (do you have duplicate srcs entries?)",
 				label, locationLabels[label], loc)
 		}
 	}