all: Tweak error creation (#6391)

- In the few places where we wrap errors, use the new Go 1.13 "%w"
  construction instead of %s or %v.

- Where we create errors with constant strings, consistently use
  errors.New and not fmt.Errorf.

- Remove capitalization from errors in the few places where we had that.

lib/fs/basicfs_windows.go

@@ -11,7 +11,6 @@ package fs
 import (
 	"bytes"
 	"errors"
-	"fmt"
 	"os"
 	"path/filepath"
 	"strings"
@@ -137,7 +136,7 @@ func (f *BasicFilesystem) Roots() ([]string, error) {
 
 	hr, _, _ := getLogicalDriveStringsHandle.Call(uintptr(unsafe.Pointer(&bufferSize)), uintptr(unsafe.Pointer(&buffer)))
 	if hr == 0 {
-		return nil, fmt.Errorf("Syscall failed")
+		return nil, errors.New("syscall failed")
 	}
 
 	var drives []string