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.go

@@ -0,0 +1,111 @@
+// 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"
+)
+
+var _ causer = (*fileError)(nil)
+
+// FileError represents an error when handling a file: Parsing a config file,
+// execute a template etc.
+type FileError interface {
+	error
+
+	// LineNumber gets the error location, starting at line 1.
+	LineNumber() int
+
+	// A string identifying the type of file, e.g. JSON, TOML, markdown etc.
+	Type() string
+}
+
+var _ FileError = (*fileError)(nil)
+
+type fileError struct {
+	lineNumber int
+	fileType   string
+	msg        string
+
+	cause error
+}
+
+func (e *fileError) LineNumber() int {
+	return e.lineNumber
+}
+
+func (e *fileError) Type() string {
+	return e.fileType
+}
+
+func (e *fileError) Error() string {
+	return e.msg
+}
+
+func (f *fileError) Cause() error {
+	return f.cause
+}
+
+func (e *fileError) Format(s fmt.State, verb rune) {
+	switch verb {
+	case 'v':
+		fallthrough
+	case 's':
+		fmt.Fprintf(s, "%s:%d: %s:%s", e.fileType, e.lineNumber, e.msg, e.cause)
+	case 'q':
+		fmt.Fprintf(s, "%q:%d: %q:%q", e.fileType, e.lineNumber, e.msg, e.cause)
+	}
+}
+
+// NewFileError creates a new FileError.
+func NewFileError(fileType string, lineNumber int, msg string, err error) FileError {
+	return &fileError{cause: err, fileType: fileType, lineNumber: lineNumber, msg: msg}
+}
+
+// UnwrapFileError tries to unwrap a FileError from err.
+// It returns nil if this is not possible.
+func UnwrapFileError(err error) FileError {
+	for err != nil {
+		switch v := err.(type) {
+		case FileError:
+			return v
+		case causer:
+			err = v.Cause()
+		default:
+			return nil
+		}
+	}
+	return nil
+}
+
+// ToFileError will try to convert the given error to an error supporting
+// the FileError interface.
+// If will fall back to returning the original error if a line number cannot be extracted.
+func ToFileError(fileType string, err error) error {
+	return ToFileErrorWithOffset(fileType, err, 0)
+}
+
+// ToFileErrorWithOffset will try to convert the given error to an error supporting
+// the FileError interface. It will take any line number offset given into account.
+// If will fall back to returning the original error if a line number cannot be extracted.
+func ToFileErrorWithOffset(fileType string, err error, offset int) error {
+	for _, handle := range lineNumberExtractors {
+		lno, msg := handle(err, offset)
+		if lno > 0 {
+			return NewFileError(fileType, lno, msg, err)
+		}
+	}
+	// Fall back to the original.
+	return err
+}