stubbs out errors, adds mask and unmask to scope

errors.go

@@ -0,0 +1,14 @@
+package systemctl
+
+import (
+	"errors"
+)
+
+var (
+	// ErrSystemctlNotInstalled means that upon trying to manually locate systemctl in the user's path,
+	// it was not found. If this error occurs, the library isn't entirely useful.
+	ErrNotInstalled = errors.New("systemd binary was not found")
+
+	// ErrExecTimeout means that the provided context was done before the command finished execution.
+	ErrExecTimeout = errors.New("command timed out")
+)