Resolve "Segmentation fault through plugin client"

A response of grpc calls within methods of DeviceModelClient is nil as soon as an error occured within the call.

In some methods values of the response were directly accessed and returned. This leads to a segmentation fault if the response is nil.

Description

If a grpc call returns an error the error is now catched and properly returned.

Related Issue

#331

Motivation and Context

How Has This Been Tested?

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:

• 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.