Prefix error messages with mc: part of #286

errors.go

@@ -18,6 +18,14 @@ package main
 
 import "fmt"
 
+type errInvalidTheme struct {
+	theme string
+}
+
+func (e errInvalidTheme) Error() string {
+	return "invalid theme: " + e.theme
+}
+
 type errInvalidArgument struct{}
 
 func (e errInvalidArgument) Error() string {