Subscriptions Fixes 1

[SebastianSchildt]

Fixes nothing in #263 , but makes subscriptions a little more robust

• enforcing string type subscriptionIDs both directions
  • fixes the occasional "Integer Overflow" on unsubscribe
• JSON schema checking for subscribe and unsubscribe requests
• as a result, overall more helpful error messages

@nayakned please check if it, at the very least, does not make things worse :)

[nayakned]

Looks good. These changes fix a part of #261.
Maybe also fix the error code returned in JSON object or document that it will be an int.
Then we can also close #261.

[SebastianSchildt]

Now all error codes are returned as string. This will fix #261 , please check @nayakned

[nayakned]

Checked a couple of errors. Those were strings as expected.