Capture error message from experimental API response

[gschneider-r7]

Description

Capture the error message from experimental API responses.

Example output (names have been changed to protect the innocent):

Nexpose::APIError: NexposeAPI: PUT request to /my/experimental/endpoint failed. response message: The thing supplied does not exist.
	from /Users/gschneider/.rvm/gems/ruby-2.4.0/gems/nexpose-6.0.0/lib/nexpose/ajax.rb:170:in `request'
	from /Users/gschneider/.rvm/gems/ruby-2.4.0/gems/nexpose-6.0.0/lib/nexpose/ajax.rb:49:in `put'

Motivation and Context

The experimental errors have a nice response and the request body is not really useful for debugging.

How Has This Been Tested?

Built the gem locally and tested a specific endpoint that I was having trouble with.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• I have updated the documentation accordingly (if changes are required).
• I have added tests to cover my changes.
• All new and existing tests passed.