Optional generation of Images extension. Make extensionOutputPath optional for Images and Colors cause it's not needed if no extensionName are provided

Sources/ResgenSwift/Images/ImagesError.swift

@@ -18,6 +18,7 @@ enum ImagesError: Error {
     case magickConvertNotFound
     case writeFile(String, String)
     case createAssetFolder(String)
+    case missingExtensionPath
     case unknown(String)
 
     var description: String {
@@ -49,6 +50,9 @@ enum ImagesError: Error {
         case .createAssetFolder(let folder):
             return "error: [\(Colors.toolName)] An error occured while creating folder `\(folder)`"
 
+        case .missingExtensionPath:
+            return "error: [\(Colors.toolName)] Extension need to be generated but no `extensionOutputPath` is provided"
+
         case .unknown(let errorDescription):
             return "error: [\(Images.toolName)] Unknown error: \(errorDescription)"
         }