feat(Document): throw error on null datacontract

[Alex-Werner]

Issue being fixed or feature implemented

If a contractId is invalid (or outdated like in the case of the bug referenced), the type locator of an app will fail with an error Cannot read property 'isDocumentDefined' of null. This PR brings more clarity so user can understand the root cause and adapt by passing another contractId to the SDK instance.

Fixes #175.

What was done?

• feat: throw error on null dataContract on creation.

How Has This Been Tested?

N/A

Breaking Changes

N/A

Checklist:

• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have added or updated relevant unit/integration/functional/e2e tests
• [X] I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• [X] I have assigned this pull request to a milestone

[shumkov]

Is this PR fixes a problem in master? If so, we need to change the base branch to master then.