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

Tedious, but I see no better way...

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