Copy errors & fs packages from the restic repo and fix import paths

Tedious, but I see no better way...

errors/doc.go

@@ -0,0 +1,2 @@
+// Package errors provides custom error types used within restic.
+package errors

errors/fatal.go

@@ -0,0 +1,38 @@
+package errors
+
+import "fmt"
+
+// fatalError is an error that should be printed to the user, then the program
+// should exit with an error code.
+type fatalError string
+
+func (e fatalError) Error() string {
+	return string(e)
+}
+
+func (e fatalError) Fatal() bool {
+	return true
+}
+
+// Fataler is an error which should be printed to the user directly.
+// Afterwards, the program should exit with an error.
+type Fataler interface {
+	Fatal() bool
+}
+
+// IsFatal returns true if err is a fatal message that should be printed to the
+// user. Then, the program should exit.
+func IsFatal(err error) bool {
+	e, ok := err.(Fataler)
+	return ok && e.Fatal()
+}
+
+// Fatal returns a wrapped error which implements the Fataler interface.
+func Fatal(s string) error {
+	return Wrap(fatalError(s), "Fatal")
+}
+
+// Fatalf returns an error which implements the Fataler interface.
+func Fatalf(s string, data ...interface{}) error {
+	return fatalError(fmt.Sprintf(s, data...))
+}

errors/wrap.go

@@ -0,0 +1,20 @@
+package errors
+
+import "github.com/pkg/errors"
+
+// Cause returns the cause of an error.
+func Cause(err error) error {
+	return errors.Cause(err)
+}
+
+// New creates a new error based on message. Wrapped so that this package does
+// not appear in the stack trace.
+var New = errors.New
+
+// Errorf creates an error based on a format string and values. Wrapped so that
+// this package does not appear in the stack trace.
+var Errorf = errors.Errorf
+
+// Wrap wraps an error retrieved from outside of restic. Wrapped so that this
+// package does not appear in the stack trace.
+var Wrap = errors.Wrap