Optional generation of extension (R.xx)

Optional generation of Colors/Fonts/Images/Stringium extension

Sources/ResgenSwift/Fonts/FontsToolError.swift

@@ -14,6 +14,7 @@ enum FontsToolError: Error {
     case inputFolderNotFound(String)
     case fileNotExists(String)
     case writeExtension(String, String)
+    case missingExtensionPath
 
     var description: String {
         switch self {
@@ -31,6 +32,9 @@ enum FontsToolError: Error {
 
         case let .writeExtension(filename, info):
             return "error: [\(Fonts.toolName)] An error occured while writing extension in \(filename): \(info)"
+
+        case .missingExtensionPath:
+            return "error: [\(Fonts.toolName)] Extension need to be generated but no `extensionOutputPath` is provided"
         }
     }
 }