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

tpl/tplimpl/template_errors.go

@@ -0,0 +1,46 @@
+// 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 tplimpl
+
+import (
+	"github.com/gohugoio/hugo/common/herrors"
+	"github.com/pkg/errors"
+	"github.com/spf13/afero"
+)
+
+type templateInfo struct {
+	template string
+
+	// Used to create some error context in error situations
+	fs afero.Fs
+
+	// The filename relative to the fs above.
+	filename string
+
+	// The real filename (if possible). Used for logging.
+	realFilename string
+}
+
+func (info templateInfo) errWithFileContext(what string, err error) error {
+	err = errors.Wrapf(err, "file %q: %s:", info.realFilename, what)
+
+	err, _ = herrors.WithFileContextForFile(
+		err,
+		info.filename,
+		info.fs,
+		"go-html-template",
+		herrors.SimpleLineMatcher)
+
+	return err
+}