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/file_error_test.go

@@ -0,0 +1,56 @@
+// 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 herrors
+
+import (
+	"errors"
+	"fmt"
+	"strconv"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestToLineNumberError(t *testing.T) {
+	t.Parallel()
+
+	assert := require.New(t)
+
+	for i, test := range []struct {
+		in         error
+		offset     int
+		lineNumber int
+	}{
+		{errors.New("no line number for you"), 0, -1},
+		{errors.New(`template: _default/single.html:2:15: executing "_default/single.html" at <.Titles>: can't evaluate field`), 0, 2},
+		{errors.New("parse failed: template: _default/bundle-resource-meta.html:11: unexpected in operand"), 0, 11},
+		{errors.New(`failed:: template: _default/bundle-resource-meta.html:2:7: executing "main" at <.Titles>`), 0, 2},
+		{errors.New("error in front matter: Near line 32 (last key parsed 'title')"), 0, 32},
+		{errors.New("error in front matter: Near line 32 (last key parsed 'title')"), 2, 34},
+	} {
+
+		got := ToFileErrorWithOffset("template", test.in, test.offset)
+
+		errMsg := fmt.Sprintf("[%d][%T]", i, got)
+		le, ok := got.(FileError)
+
+		if test.lineNumber > 0 {
+			assert.True(ok)
+			assert.Equal(test.lineNumber, le.LineNumber(), errMsg)
+			assert.Contains(got.Error(), strconv.Itoa(le.LineNumber()))
+		} else {
+			assert.False(ok)
+		}
+	}
+}