Improve error messages, esp. when the server is running

* Add file context to minifier errors when publishing
* Misc fixes (see issues)
* Allow custom server error template in layouts/server/error.html

To get to this, this commit also cleans up and simplifies the code surrounding errors and files. This also removes the usage of `github.com/pkg/errors`, mostly because of https://github.com/pkg/errors/issues/223 -- but also because most of this is now built-in to Go.

Fixes #9852
Fixes #9857
Fixes #9863

common/herrors/errors.go

@@ -19,37 +19,11 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"os"
 	"runtime"
 	"runtime/debug"
 	"strconv"
-
-	_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
-}
-
-// PrintStackTraceFromErr prints the error's stack trace to stdoud.
-func PrintStackTraceFromErr(err error) {
-	FprintStackTraceFromErr(os.Stdout, err)
-}
-
-// FprintStackTraceFromErr prints the error's stack trace to w.
-func FprintStackTraceFromErr(w io.Writer, err error) {
-	if err, ok := err.(stackTracer); ok {
-		for _, f := range err.StackTrace() {
-			fmt.Fprintf(w, "%+s:%d\n", f, f)
-		}
-	}
-}
-
 // PrintStackTrace prints the current stacktrace to w.
 func PrintStackTrace(w io.Writer) {
 	buf := make([]byte, 1<<16)