[errors] Add built in ErrnoError

[NoHomey]

• Version: 6.8.1
• Platform: Linux nohomey 4.4.0-43-generic ee.getMaxListeners() #63-Ubuntu SMP Wed Oct 12 13:48:03 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
• Subsystem: util, spawn, dns .... (all modules that throw the so known System Error)

System Errors are documented in the API Documentation and they are thrown all over the code, such examples are: util, spawn and dns. So I would like to propose to add built in ErrnoError class that can be used both by addon developers, node and even some user JavaScript code and to deprecate the current System Errors which are just augmented Errors. ErrnoError constructor may accept two arguments errno of type int and syscall of type string and it should generate proper error message like system call X failed with Y errno code, {human readable meaning}. ErrnoError may also be separated in it's own header/source where errno codes are also mapped and as well available to addon writes then exposed to JavaScript, which is currently done in node_constants.
And finally why I want such change to be made: to remove redundancy, as a node addon developer some times I need to define a macro that throws System Errors and there is no built in one so I have to either re-use the implementation from my previous project or created new one that slightly differs from the previous one - there is no standart way how Error instances are augmented, this redundancy can even be found in node's code itself: again in util, spawn and dns but is in JavaScript code not C++ ...

[bnoordhuis]

What you are asking for exists and has for a long time: node::ErrnoException() from src/node.h. I'm going to go ahead and close this.

[NoHomey]

@bnoordhuis I've checked node::ErrnoException and found that it slightly differs from the documented System Error in that, it sets errno to the passed errorno instead of it's string representation, like code to estring. In node's source ThrowErrnoException is used to throw ErrnoExceptions and I can't find a single place in node's source (C++) where the thrown ErrnoException is caught and it's errno is properly set to the string representation. However I do find places where this is done from the JavaScript code: (going to repeat my self with the examples from above: util, spawn and dns).

Is setting errno to the actual errno the correct behavior?

If not I do found this to be big redundancy since errno could be set directly to the same as code from node::ErrnoException and I would like to make a pull request to fix this issue in such case.

[bnoordhuis]

Is setting errno to the actual errno the correct behavior?

Yes. If anything, the inconsistencies are in lib/ because we don't always set .code and .errno to a string and a number respectively.