Added downloading mods from Modrinth

src/errors/download_error.ts

@@ -0,0 +1,7 @@
+export default class DownloadError extends Error {
+    constructor(message: string | undefined) {
+        super(message);
+        this.name = this.constructor.name;
+        Error.captureStackTrace(this, this.constructor);
+    }
+}

src/errors/mod_not_found_error.ts

@@ -0,0 +1,7 @@
+export default class ModNotFoundError extends Error {
+    constructor(message: string) {
+        super(message);
+        this.name = this.constructor.name;
+        Error.captureStackTrace(this, this.constructor);
+    }
+}