Produce a more useful error when flag-like values are used for flag values.

eg.  myapp: error: --log-level: expected string value but got "--foo" (long flag); perhaps try --log-level="--foo"?

scanner.go

@@ -3,8 +3,6 @@ package kong
 import (
 	"fmt"
 	"strings"
-
-	"github.com/pkg/errors"
 )
 
 // TokenType is the type of a token.
@@ -14,11 +12,11 @@ type TokenType int
 const (
 	UntypedToken TokenType = iota
 	EOLToken
-	FlagToken                // --<flag>
-	FlagValueToken           // =<value>
-	ShortFlagToken           // -<short>[<tail]
-	ShortFlagTailToken       // <tail>
-	PositionalArgumentToken  // <arg>
+	FlagToken               // --<flag>
+	FlagValueToken          // =<value>
+	ShortFlagToken          // -<short>[<tail]
+	ShortFlagTailToken      // <tail>
+	PositionalArgumentToken // <arg>
 )
 
 func (t TokenType) String() string {
@@ -142,13 +140,22 @@ func (s *Scanner) Pop() Token {
 	return arg
 }
 
+type expectedError struct {
+	context string
+	token   Token
+}
+
+func (e *expectedError) Error() string {
+	return fmt.Sprintf("expected %s value but got %q (%s)", e.context, e.token, e.token.InferredType())
+}
+
 // PopValue pops a value token, or returns an error.
 //
 // "context" is used to assist the user if the value can not be popped, eg. "expected <context> value but got <type>"
 func (s *Scanner) PopValue(context string) (Token, error) {
 	t := s.Pop()
 	if !t.IsValue() {
-		return t, errors.Errorf("expected %s value but got %q (%s)", context, t, t.InferredType())
+		return t, &expectedError{context, t}
 	}
 	return t, nil
 }
@@ -157,9 +164,9 @@ func (s *Scanner) PopValue(context string) (Token, error) {
 //
 // "context" is used to assist the user if the value can not be popped, eg. "expected <context> value but got <type>"
 func (s *Scanner) PopValueInto(context string, target interface{}) error {
-	t := s.Pop()
-	if !t.IsValue() {
-		return errors.Errorf("expected %s value but got %q (%s)", context, t, t.InferredType())
+	t, err := s.PopValue(context)
+	if err != nil {
+		return err
 	}
 	return jsonTranscode(t.Value, target)
 }