Explore other error conditions for fetching a Thing Description

[relu91]

    Here is also a good place to put the same error condition as specified in the `exploreDirectory` method:

if the underlying
implementation cannot support the Protocol Binding
indicated by |url|

Originally posted by @relu91 in #441 (comment)

[zolkis]

The problem here is that

• we have a section for fetching a TD that refers to HTTP fetch or libraries that implement fetching over other protocols as well;
• the algorithm in the issue has a text like

If fetching a Thing Description is not supported by the implementation, ...

which is under-specified. We should have an algorithm for that, but it has been very tricky and there are good reasons why we don't have it. Fetch is a big spec itself, and that's only HTTP/S. Adding more protocols blows complexity upwards. That is outside the scope of the WoT Scripting spec (which is basically just a tad more than documentation for a library). We need to be able to describe choosing Protocol Bindings in terms of scripting algorithms, point to WoT and external algorithms from other specs etc.