container/init: unwrap descriptive fatal error

These errors are printed with a descriptive message prefixed to them, so it is more readable to expose the underlying errno.

Signed-off-by: Ophestra <cat@gensokyo.uk>

container/errors.go

@@ -59,6 +59,7 @@ func messagePrefixP[V any, T interface {
 	return zeroString, false
 }
 
+// MountError wraps errors returned by syscall.Mount.
 type MountError struct {
 	Source, Target, Fstype string
 
@@ -74,6 +75,7 @@ func (e *MountError) Unwrap() error {
 	return e.Errno
 }
 
+func (e *MountError) Message() string { return "cannot " + e.Error() }
 func (e *MountError) Error() string {
 	if e.Flags&syscall.MS_BIND != 0 {
 		if e.Flags&syscall.MS_REMOUNT != 0 {
@@ -90,6 +92,15 @@ func (e *MountError) Error() string {
 	return "mount " + e.Target + ": " + e.Errno.Error()
 }
 
+// optionalErrorUnwrap calls [errors.Unwrap] and returns the resulting value
+// if it is not nil, or the original value if it is.
+func optionalErrorUnwrap(err error) error {
+	if underlyingErr := errors.Unwrap(err); underlyingErr != nil {
+		return underlyingErr
+	}
+	return err
+}
+
 // errnoFallback returns the concrete errno from an error, or a [os.PathError] fallback.
 func errnoFallback(op, path string, err error) (syscall.Errno, *os.PathError) {
 	var errno syscall.Errno