More explicit exception should be thrown when continuation token is invalid

[jackliums]

Describe the bug
If an invalid continuation token is passed in, the server throws generic 500 Internal Server Error. We should throw more specific error here.

To Reproduce
Steps to reproduce the behavior:

1. Make a search call with some invalid continuation token.

Expected behavior
The server throw an error indicating that the continuation token is invalid.

Actual behavior
500 Internal Server Error is thrown with generic error message.