Add LibUsbError and InitFalied error variant. new returns Result. Bum…

[cr1901]

…p version.

Rationale:

• As opposed to a string, LibUsbError has private fields which can change without breaking semver.
• Fallible construction is a reasonable API design. c.f. File::open. Add a variant to deal with that particular failure.
• Since all libftdi1 strings are ASCII, converting to UTF8 genuinely won't fail and I'd prefer not to panic in a library (though unreachable! already prevents that).
• bindgen feature is exposed- disabled by default.

[tanriol]

Bundling all of this in one commit is not a good idea :-)
Also, I don't like the idea of every builder step being fallible. If you insist on handling the context initialization failure, I'd prefer to defer creating the actual context until the final step.

[cr1901]

@tanriol I will fix after the weekend; I need a break, otherwise I risk burning out with the other things I'm working on.

[tanriol]

Sure, no hurry.

[cr1901]

@tanriol I'm in a much better headspace now after a break. I'll work on incorporating your suggestions into the non-draft PRs later today.

Also, I sent an email discussing some further developments; I think I should keep you in the loop.