WebAPI: Respond with more detailed info

* WebAPI: return error message when endpoint not found
* WebAPI: send appropriate status code when logging in
* WebAPI: return more info when adding torrents

PR #23202.
Closes #375.
Closes #10688.
Closes #10747.
Closes #11464.

src/webui/webapplication.cpp

@@ -418,6 +418,8 @@ void WebApplication::doProcessRequest()
             throw ConflictHTTPError(error.message());
         case APIErrorType::NotFound:
             throw NotFoundHTTPError(error.message());
+        case APIErrorType::Unauthorized:
+            throw UnauthorizedHTTPError(error.message());
         default:
             Q_UNREACHABLE();
             break;