fix: 500/ dont expose invalid base64 decode error for page token

[markphelps]

Fixes: FLI-189

• Prevent 500
• Don't expose invalid base64 decode error to user if bad pageToken

Before

curl -v "http://localhost:8080/api/v1/flags/flagKey/rules?limit=10&offset=10&pageToken=pageToken"

< HTTP/1.1 500 Internal Server Error
< Content-Type: application/json
< Vary: Origin
< X-Content-Type-Options: nosniff
< Date: Fri, 27 Jan 2023 20:26:02 GMT
< Content-Length: 72

{"code":13,"message":"illegal base64 data at input byte 8","details":[]}%     

After

curl -v "http://localhost:8080/api/v1/flags/flagKey/rules?limit=10&offset=10&pageToken=pageToken"

< HTTP/1.1 400 Bad Request
< Content-Type: application/json
< Vary: Origin
< Date: Thu, 02 Feb 2023 21:16:11 GMT
< Content-Length: 70

{"code":3,"message":"pageToken is not valid: \"pageToken\"","details":[]}%     

[GeorgeMac]

Lovely.