fix: send a meaningful error response in case an answer/topic can't be fetched

sometimes URL is malinformed(eg: accessing topic/some-name instead of profile/some-name).
other times it's Quora that doesn't populate their JSON.
In all these cases, fetcher script returns an empty object.
So, sending meaningful error messages might help the user.

README.md

@@ -150,7 +150,7 @@ From [their privacy policy](https://www.quora.com/about/privacy)
 
   Quora doesn't show all answers at once. It only loads more answers as the user scrolls down. Furthermore, it uses many unique IDs to send ajax requests to fetch those answers. So, all in all, getting more answers isn't impossible but quite difficult requiring some serious amount of time on their website in order to figure out how it all happens. I'm short on time for now.
 
-- Why am I getting a _Couldn't find such an answer_ error?
+- Why am I getting a _Recheck the URL_ error?
 
   Sometimes Quora doesn't populate the answer page HTML, and hence, Quetre is unable to extract data from it. If that happens, you can refresh the page a couple of times to get the answers.
 

fetchers/getAnswers.js

@@ -18,8 +18,8 @@ const getAnswers = async slug => {
 
   if (!rawData)
     throw new AppError(
-      "Couldn't find such an answer. Maybe check the URL?",
-      400
+      "Answers couldn't be fetched. Recheck the URL, or resend the request if you believe the URL is correct.",
+      404
     );
 
   // array containing all the answers with metadata

fetchers/getTopic.js

@@ -16,7 +16,10 @@ const getTopic = async slug => {
   } = JSON.parse(res);
 
   if (!rawData)
-    throw new AppError("couldn't find such a topic. Maybe check the URL?", 400);
+    throw new AppError(
+      "Topic details couldn't be fetched. Recheck the URL, or resend the request if you believe the URL is correct.",
+      404
+    );
 
   const data = {
     tid: rawData.tid,