Describe error response format for directory API

[farshidtz]

This describes the use of Problem Details for response errors. It also adds a short intro before TD object.

It includes the following editorial note:

WoT-specific error types
The Problem Details error type field is a URI reference which could used to map the occurred error to WoT-specific error class. There are few open issues raising the lack of WoT-specific error types: wot-discovery#44, wot-thing-description#303, wot-scripting-api#200.
For now, type can be omitted which defaults to "about:blank", and title should be set to HTTP status text.

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[mmccool]

As discussed in the meeting, we need to add some details of error responses in the TD (including different content types for error responses than the successful response) but this will be done in an additional PR, and we will merge this one as a baseline.