Improved assembler

src/errors/InvalidDirectory.go

@@ -2,6 +2,7 @@ package errors
 
 import "fmt"
 
+// InvalidDirectory errors are returned when the specified path is not a directory.
 type InvalidDirectory struct {
 	Path string
 }

src/errors/RegisterInUse.go

@@ -0,0 +1,14 @@
+package errors
+
+import "fmt"
+
+// RegisterInUse errors are returned when a register is already in use.
+type RegisterInUse struct {
+	Register string
+	User     string
+}
+
+// Error implements the text representation.
+func (err *RegisterInUse) Error() string {
+	return fmt.Sprintf("Register '%s' already used by '%s'", err.Register, err.User)
+}