Do not throw exceptions in Repository

[JaZo]

Description

I removed the exceptions from Repository and added tests for it.

Motivation and context

The repository only throwd exceptions in some use cases (GET requests) which was not consistent with other use cases. Besides, the repository consumer might want to handle errors differently, but because of the current implementation, he/she has no access to the underlying document. By removing the exceptions, the consumer can implement any check he/she wants. The old behaviour can be recreated by simply extending the repository and implementing the checks yourself. This closes #39.

How has this been tested?

Tested using new unit tests.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• 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:

Go over all the following points, and put an x in all the boxes that apply.

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.