Add missing errno constants

[j8r]

That's mostly a POC to improve the error handling around Errno, as pointed by @bararchy on Gitter.

Content of this PR

• Reordering the LibC Errno constants to facilitate their read.
• Add missing Errno constants (which also make the language compiling for the next commit).
• Add the Error::Errno exception class.

Goal

Having different error class for each Errno allows to rescue them, instead of having to compare their integer values with ==.
The end user would also be able to figure out what's the error name instead of having the Errno class with #errno as an integer.

What's next

There are still places where the Errno::Error is exposed directly instead of a subclass.

This is an important breaking change, we have to weigh carefully the pros/cons before any action.

[ysbaddaden]

Having different classes for Errno was rejected years ago. It creates lots of classes and increased compilation by a non negligible factor.

All I see here is noise (don't mess with libc please) and confusion around Errno::Error vs the Errno Enum for the time being.

[straight-shoota]

This topic should be discussed in an issue, not a PR.

And we're certainly not going to map each errno value to a separate error type.
But we could replace Errno with a number of more specific error classes, which would essentially group related errnos into one type. This has been proposed before, but never really discussed. Related discussions are in #1124 and #4835. Please open an issue for this.

[ysbaddaden]

See #445 (quite a small number) where that was implemented and rejected.

[straight-shoota]

@ysbaddaden #445 was duplicating each errno constant as a distinct type (same as this PR). That's not going to work, yes. But an alternative approach as described in #1124 (comment) would be worth looking at.