commands: Show server error info in browser

The main item in this commit is showing of errors with a file context when running `hugo server`.

This can be turned off: `hugo server --disableBrowserError` (can also be set in `config.toml`).

But to get there, the error handling in Hugo needed a revision. There are some items left TODO for commits soon to follow, most notable errors in content and config files.

Fixes #5284
Fixes #5290
See #5325
See #5324

common/herrors/line_number_extractors.go

@@ -0,0 +1,59 @@
+// 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.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitatio	ns under the License.
+
+package herrors
+
+import (
+	"fmt"
+	"regexp"
+	"strconv"
+)
+
+var lineNumberExtractors = []lineNumberExtractor{
+	// Template/shortcode parse errors
+	newLineNumberErrHandlerFromRegexp("(.*?:)(\\d+)(:.*)"),
+
+	// TOML parse errors
+	newLineNumberErrHandlerFromRegexp("(.*Near line )(\\d+)(\\s.*)"),
+
+	// YAML parse errors
+	newLineNumberErrHandlerFromRegexp("(line )(\\d+)(:)"),
+}
+
+type lineNumberExtractor func(e error, offset int) (int, string)
+
+func newLineNumberErrHandlerFromRegexp(expression string) lineNumberExtractor {
+	re := regexp.MustCompile(expression)
+	return extractLineNo(re)
+}
+
+func extractLineNo(re *regexp.Regexp) lineNumberExtractor {
+	return func(e error, offset int) (int, string) {
+		if e == nil {
+			panic("no error")
+		}
+		s := e.Error()
+		m := re.FindStringSubmatch(s)
+		if len(m) == 4 {
+			i, _ := strconv.Atoi(m[2])
+			msg := e.Error()
+			if offset != 0 {
+				i = i + offset
+				msg = re.ReplaceAllString(s, fmt.Sprintf("${1}%d${3}", i))
+			}
+			return i, msg
+		}
+
+		return -1, ""
+	}
+}