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_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()