Ensure ParseError.Message is always set (#411)

ParseError.Error() did:

	msg := pe.Message
	if msg == "" {
		msg = pe.err.Error()
	}

That was fine, but not very useful for people wanting to access the
Message field themselves, especially since err isn't exported.

For example staticcheck does this to add in the filename, and the error
is lost, because Message is often blank:

	% staticcheck
	staticcheck.conf:5:0:  (last key parsed: "dot_import_whitelist") (compile)

We now only use the err field to determine which error usage to display
in ErrorWithUsage()

decode.go

@@ -542,10 +542,11 @@ func (md *MetaData) parseErr(err error) error {
 	k := md.context.String()
 	d := string(md.data)
 	return ParseError{
+		Message:  err.Error(),
+		err:      err,
 		LastKey:  k,
 		Position: md.keyInfo[k].pos.withCol(d),
 		Line:     md.keyInfo[k].pos.Line,
-		err:      err,
 		input:    d,
 	}
 }

error.go

@@ -92,16 +92,11 @@ func (p Position) withCol(tomlFile string) Position {
 }
 
 func (pe ParseError) Error() string {
-	msg := pe.Message
-	if msg == "" { // Error from errorf()
-		msg = pe.err.Error()
-	}
-
 	if pe.LastKey == "" {
-		return fmt.Sprintf("toml: line %d: %s", pe.Position.Line, msg)
+		return fmt.Sprintf("toml: line %d: %s", pe.Position.Line, pe.Message)
 	}
 	return fmt.Sprintf("toml: line %d (last key %q): %s",
-		pe.Position.Line, pe.LastKey, msg)
+		pe.Position.Line, pe.LastKey, pe.Message)
 }
 
 // ErrorWithPosition returns the error with detailed location context.
@@ -112,25 +107,19 @@ func (pe ParseError) ErrorWithPosition() string {
 		return pe.Error()
 	}
 
+	// TODO: don't show control characters as literals? This may not show up
+	// well everywhere.
+
 	var (
 		lines = strings.Split(pe.input, "\n")
 		b     = new(strings.Builder)
 	)
-
-	msg := pe.Message
-	if msg == "" {
-		msg = pe.err.Error()
-	}
-
-	// TODO: don't show control characters as literals? This may not show up
-	// well everywhere.
-
 	if pe.Position.Len == 1 {
 		fmt.Fprintf(b, "toml: error: %s\n\nAt line %d, column %d:\n\n",
-			msg, pe.Position.Line, pe.Position.Col)
+			pe.Message, pe.Position.Line, pe.Position.Col)
 	} else {
 		fmt.Fprintf(b, "toml: error: %s\n\nAt line %d, column %d-%d:\n\n",
-			msg, pe.Position.Line, pe.Position.Col, pe.Position.Col+pe.Position.Len-1)
+			pe.Message, pe.Position.Line, pe.Position.Col, pe.Position.Col+pe.Position.Len-1)
 	}
 	if pe.Position.Line > 2 {
 		fmt.Fprintf(b, "% 7d | %s\n", pe.Position.Line-2, expandTab(lines[pe.Position.Line-3]))

parse.go

@@ -92,6 +92,7 @@ func parse(data string) (p *parser, err error) {
 
 func (p *parser) panicErr(it item, err error) {
 	panic(ParseError{
+		Message:  err.Error(),
 		err:      err,
 		Position: it.pos.withCol(p.lx.input),
 		Line:     it.pos.Len,
@@ -123,10 +124,11 @@ func (p *parser) next() item {
 	if it.typ == itemError {
 		if it.err != nil {
 			panic(ParseError{
+				Message:  it.err.Error(),
+				err:      it.err,
 				Position: it.pos.withCol(p.lx.input),
 				Line:     it.pos.Line,
 				LastKey:  p.current(),
-				err:      it.err,
 			})
 		}