Verification module is undefined if the signing secret key is not provided

[liangyuanruo]

The runtime SDK has the strange behavior where the verification module is missing if the signing secret key isn’t provided.

Initially thought to be a smart way to protect the SDK user, this design actually causes the need for type coercion in the verification service within the main Forms codebase (see opengovsg/FormSG#941).

Change the behavior of the SDK to throw an exception instead.

[frankchn]

I suspect I am probably missing something here, but seems like the Verification module is always constructed upon package initialization, and the current code does throw an Exception when the verification module is being used without a secret key.

[karrui]

Yes, this should have been fixed in #38. This is just not pushed to npm yet