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/errors.go

@@ -0,0 +1,53 @@
+// 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 contains common Hugo errors and error related utilities.
+package herrors
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+
+	_errors "github.com/pkg/errors"
+)
+
+// As defined in https://godoc.org/github.com/pkg/errors
+type causer interface {
+	Cause() error
+}
+
+type stackTracer interface {
+	StackTrace() _errors.StackTrace
+}
+
+// PrintStackTrace prints the error's stack trace to stdoud.
+func PrintStackTrace(err error) {
+	FprintStackTrace(os.Stdout, err)
+}
+
+// FprintStackTrace prints the error's stack trace to w.
+func FprintStackTrace(w io.Writer, err error) {
+	if err, ok := err.(stackTracer); ok {
+		for _, f := range err.StackTrace() {
+			fmt.Fprintf(w, "%+s:%d\n", f, f)
+		}
+	}
+}
+
+// ErrFeatureNotAvailable denotes that a feature is unavailable.
+//
+// We will, at least to begin with, make some Hugo features (SCSS with libsass) optional,
+// and this error is used to signal those situations.
+var ErrFeatureNotAvailable = errors.New("this feature is not available in your current Hugo version")