Support concrete errors / pointers in tuple returns too.

Fixes #80

internal/errcheck/errcheck.go

@@ -237,8 +237,16 @@ func (v *visitor) errorsByArg(call *ast.CallExpr) []bool {
 		// Multiple returns
 		s := make([]bool, t.Len())
 		for i := 0; i < t.Len(); i++ {
-			nt, ok := t.At(i).Type().(*types.Named)
-			s[i] = ok && isErrorType(nt)
+			switch et := t.At(i).Type().(type) {
+			case *types.Named:
+				// Single return
+				s[i] = isErrorType(et)
+			case *types.Pointer:
+				// Single return via pointer
+				s[i] = isErrorType(et)
+			default:
+				s[i] = false
+			}
 		}
 		return s
 	}

main_test.go

@@ -54,7 +54,7 @@ func TestMain(t *testing.T) {
 		t.Errorf("Exit code is %d, expected %d", exitCode, exitUncheckedError)
 	}
 
-	expectUnchecked := 12
+	expectUnchecked := 14
 	if got := strings.Count(out, "UNCHECKED"); got != expectUnchecked {
 		t.Errorf("Got %d UNCHECKED errors, expected %d in:\n%s", got, expectUnchecked, out)
 	}

testdata/main.go

@@ -39,6 +39,10 @@ func customConcreteError() MyError {
 	return MyError("an error occurred")
 }
 
+func customConcreteErrorTuple() (int, MyError) {
+	return 0, MyError("an error occurred")
+}
+
 type MyPointerError string
 
 func (e *MyPointerError) Error() string {
@@ -50,6 +54,11 @@ func customPointerError() *MyPointerError {
 	return &e
 }
 
+func customPointerErrorTuple() (int, *MyPointerError) {
+	e := MyPointerError("an error occurred")
+	return 0, &e
+}
+
 func main() {
 	// Single error return
 	_ = a() // BLANK
@@ -64,12 +73,16 @@ func main() {
 	customError()     // UNCHECKED
 
 	// Return a custom concrete error type
-	_ = customConcreteError() // BLANK
-	customConcreteError()     // UNCHECKED
+	_ = customConcreteError()         // BLANK
+	customConcreteError()             // UNCHECKED
+	_, _ = customConcreteErrorTuple() // BLANK
+	customConcreteErrorTuple()        // UNCHECKED
 
 	// Return a custom pointer error type
-	_ = customPointerError() // BLANK
-	customPointerError()     // UNCHECKED
+	_ = customPointerError()         // BLANK
+	customPointerError()             // UNCHECKED
+	_, _ = customPointerErrorTuple() // BLANK
+	customPointerErrorTuple()        // UNCHECKED
 
 	// Method with a single error return
 	x := t{}