[language.support, utilities] Condense description of exception classes

[jensmaurer]

Fixes #748.

[jensmaurer]

Note that this currently has a lame sentence "The constructors are specified in \ref{exception}." to point readers to the paragraph that says "all standard library classes T deriving from std::exception have these constructors". I've also removed the NTBS blurb from the derived classes; it's now implied by std::exception::what(). Maybe we can get rid of the lame sentence and just add a comment in the class synopsis, saying something like "// see also \ref{exception}. Suggestions welcome.

Oh, and before I forget: This is borderline editorial; for @zygoloid to decide.

[zygoloid]

I would prefer a comment inside the class definition pointing at [exception] rather than wording in a section that a casual reader may well not look at.

Just using the defaulted special members here seems very reasonable to me. I think it's fine that we now give these exception subclasses move operations, but a second opinion on that would be good (@jwakely?).

[jensmaurer]

Updated.

The fun part is that logic_error and other classes from <stdexcept> that take a user-defined string now also have a default constructor per [exception]. That might be undesirable, and is definitely not an editorial change.

[jensmaurer]

Fixed the "logic_error now has a default constructor" issue.

We might want to have LWG ack the changes.

[jensmaurer]

Editorial meeting consensus: Looks like the right direction. @zygoloid asks for LWG expert review. @jwakely ?

[tkoeppe]

@jwakely: Ping

[jensmaurer]

Rebased.

[jensmaurer]

Rebased.

@jwakely, ping?

[jensmaurer]

@jwakely, ping^2.