[expected.object.general] Add missing noexcept for expected::error()

[hewillk]

In [expected.object.general], the function is defined as

constexpr const E& error() const &;

However in [expected.object.obs], it is defined as

constexpr const E& error() const & noexcept;

So let me assume it's just that the author of the paper forgot to add noexcept.

[jensmaurer]

@jwakely, this looks like an oversight in the incoming paper. Do you think this is editorial, or does it need an LWG issue?

Note: squash and fix commit message when merging

[jensmaurer]

@jwakely , ping.

[jwakely]

I don't recall the intention or whether we discussed making these narrow contact functions noexcept. I'll check the minutes.

[jwakely]

The noexcept on the detailed descriptions appeared in P0323R11 following the 2021-10-08 LWG review. The minutes show:

• TS: there should be noexcept on the -> and * operators although they have a precondition
• JW: we should get rid of the expected:: qualification on value()
• JW: error() can be noexcept

It looks like we agreed to add it then, but it wasn't added to the synopsis. So I think this can be fixed editorially.