fix: explicitly handle async JavaScript failures.

[DavidVujic]

Explicit checks for errors by validating the result code coming from the C Client.

Description

All calls to methods in the promisified version of the JavaScript client has now an explicit result code check. A result code that has a negative value is a failure, and the method will from now on perform a reject.

Before this, errors failed silently, as described in issue #237 - the issue is about the set method, but the error exists in all promisified method calls.

Motivation and Context

fixes #237

How Has This Been Tested?

npm tests passed
CircleCI passed
added example code that verifies the behaviour.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have read the code of conduct.
• I have read the contributing guide.
• I have updated the documentation accordingly (if applicable).