Agent extension api (#5281)

Co-authored-by: Douwe Osinga <douwe@squareup.com>

ui/desktop/src/utils/conversionUtils.ts

@@ -0,0 +1,23 @@
+export async function safeJsonParse<T>(
+  response: Response,
+  errorMessage: string = 'Failed to parse server response'
+): Promise<T> {
+  try {
+    return (await response.json()) as T;
+  } catch (error) {
+    if (error instanceof SyntaxError) {
+      throw new Error(errorMessage);
+    }
+    throw error;
+  }
+}
+
+export function errorMessage(err: Error | unknown, default_value?: string) {
+  if (err instanceof Error) {
+    return err.message;
+  } else if (typeof err === 'object' && err !== null && 'message' in err) {
+    return String(err.message);
+  } else {
+    return default_value || String(err);
+  }
+}