Fixes #9 - unify errors

[shazron]

Description

Unifies errors by using an Error object for all promise rejections.

Related Issue

See #9

Motivation and Context

For users of this lib, it will end up in cleaner code rather than trying to handle the various types of promise rejections.

How Has This Been Tested?

Passes all npm test scenarios.

Screenshots (if appropriate):

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 change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[shazron]

Tests are pending for the new errors. Existing tests pass.

[shazron]

Tests done.