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/error_locator_test.go

@@ -0,0 +1,112 @@
+// 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
+// limitations under the License.
+
+// Package errors contains common Hugo errors and error related utilities.
+package herrors
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestErrorLocator(t *testing.T) {
+	assert := require.New(t)
+
+	lineMatcher := func(le FileError, lineno int, line string) bool {
+		return strings.Contains(line, "THEONE")
+	}
+
+	lines := `LINE 1
+LINE 2
+LINE 3
+LINE 4
+This is THEONE
+LINE 6
+LINE 7
+LINE 8
+`
+
+	location := locateErrorInString(nil, lines, lineMatcher)
+	assert.Equal([]string{"LINE 3", "LINE 4", "This is THEONE", "LINE 6", "LINE 7"}, location.Lines)
+
+	assert.Equal(3, location.LineNumber)
+	assert.Equal(2, location.Pos)
+
+	assert.Equal([]string{"This is THEONE"}, locateErrorInString(nil, `This is THEONE`, lineMatcher).Lines)
+
+	location = locateErrorInString(nil, `L1
+This is THEONE
+L2
+`, lineMatcher)
+	assert.Equal(1, location.Pos)
+	assert.Equal([]string{"L1", "This is THEONE", "L2"}, location.Lines)
+
+	location = locateErrorInString(nil, `This is THEONE
+L2
+`, lineMatcher)
+	assert.Equal(0, location.Pos)
+	assert.Equal([]string{"This is THEONE", "L2"}, location.Lines)
+
+	location = locateErrorInString(nil, `L1
+This THEONE
+`, lineMatcher)
+	assert.Equal([]string{"L1", "This THEONE"}, location.Lines)
+	assert.Equal(1, location.Pos)
+
+	location = locateErrorInString(nil, `L1
+L2
+This THEONE
+`, lineMatcher)
+	assert.Equal([]string{"L1", "L2", "This THEONE"}, location.Lines)
+	assert.Equal(2, location.Pos)
+
+	location = locateErrorInString(nil, "NO MATCH", lineMatcher)
+	assert.Equal(-1, location.LineNumber)
+	assert.Equal(-1, location.Pos)
+	assert.Equal(0, len(location.Lines))
+
+	lineMatcher = func(le FileError, lineno int, line string) bool {
+		return lineno == 6
+	}
+	location = locateErrorInString(nil, `A
+B
+C
+D
+E
+F
+G
+H
+I
+J`, lineMatcher)
+
+	assert.Equal([]string{"D", "E", "F", "G", "H"}, location.Lines)
+	assert.Equal(4, location.LineNumber)
+	assert.Equal(2, location.Pos)
+
+	// Test match EOF
+	lineMatcher = func(le FileError, lineno int, line string) bool {
+		return lineno == 4
+	}
+
+	location = locateErrorInString(nil, `A
+B
+C
+`, lineMatcher)
+
+	assert.Equal([]string{"B", "C", ""}, location.Lines)
+	assert.Equal(3, location.LineNumber)
+	assert.Equal(2, location.Pos)
+
+}