herrors: Improve handling of JSON errors

`*json.UnmarshalTypeError` and `*json.SyntaxError` has a byte `Offset`, so use that.

This commit also reworks/simplifies the errror line matching logic. This also makes the file reading unbuffered, but that should be fine in this error case.

See #5324

hugolib/hugo_sites.go

@@ -1,4 +1,4 @@
-// Copyright 2016-present The Hugo Authors. All rights reserved.
+// Copyright 2018 The Hugo Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.

hugolib/hugo_sites_build_errors_test.go

@@ -150,8 +150,7 @@ func TestSiteBuildErrors(t *testing.T) {
 			name:     "Invalid YAML front matter",
 			fileType: yamlcontent,
 			fileFixer: func(content string) string {
-				// TODO(bep) 2errors YAML line numbers seems to be off by one for > 1 line.
-				return strings.Replace(content, "title:", "title", 1)
+				return strings.Replace(content, "title:", "title: %foo", 1)
 			},
 			assertBuildError: func(a testSiteBuildErrorAsserter, err error) {
 				a.assertLineNumber(2, err)
@@ -170,6 +169,20 @@ func TestSiteBuildErrors(t *testing.T) {
 
 			},
 		},
+		{
+			name:     "Invalid JSON front matter",
+			fileType: tomlcontent,
+			fileFixer: func(content string) string {
+				return strings.Replace(content, "\"description\":", "\"description\"", 1)
+			},
+			assertBuildError: func(a testSiteBuildErrorAsserter, err error) {
+				fe := a.getFileError(err)
+
+				assert.Equal(3, fe.LineNumber)
+				assert.Equal("json", fe.ErrorContext.ChromaLexer)
+
+			},
+		},
 		{
 			name:     "Panic in template Execute",
 			fileType: single,
@@ -246,6 +259,16 @@ description = "Descriptioon"
 Some content.
 
 
+`))
+
+		b.WithContent("myjson.md", f(tomlcontent, `{
+	"title": "This is a title",
+	"description": "This is a description."
+}
+
+Some content.
+
+
 `))
 
 		createErr := b.CreateSitesE()

hugolib/page_content.go

@@ -89,7 +89,11 @@ Loop:
 			f := metadecoders.FormatFromFrontMatterType(it.Type)
 			m, err := metadecoders.UnmarshalToMap(it.Val, f)
 			if err != nil {
-				return herrors.ToFileErrorWithOffset(string(f), err, iter.LineNumber()-1)
+				if fe, ok := err.(herrors.FileError); ok {
+					return herrors.ToFileErrorWithOffset(fe, iter.LineNumber()-1)
+				} else {
+					return err
+				}
 			}
 			if err := p.updateMetaData(m); err != nil {
 				return err
@@ -192,6 +196,6 @@ func parseError(err error, input []byte, pos int) error {
 	input = input[:pos]
 	lineNumber := bytes.Count(input, lf) + 1
 	endOfLastLine := bytes.LastIndex(input, lf)
-	return herrors.NewFileError("md", lineNumber, pos-endOfLastLine, err)
+	return herrors.NewFileError("md", -1, lineNumber, pos-endOfLastLine, err)
 
 }