Integrate with C++11 system_error (see PR #27)

[EvanBalster]

(Lodging an issue for this; I might be inclined to develop a pull request.)

C++11 introduces an error_code class that can accommodate both POSIX standard error codes and alternative collections of error codes such as NNG's. An error_code includes an integer value (0 for no error) and an error category which can be either the C++/POSIX one or some custom category. nngpp would benefit from implementing a custom error category.

https://en.cppreference.com/w/cpp/error/error_category
Implementations of std::error_category have a few responsibilities: Categorizing error codes, providing string representations, and mapping platform-dependent codes to platform-agnostic ones (including standard C++/POSIX error codes). These functions serve to make codebases more consistent and integrable; for example, libasio uses std::error_code extensively.

I recommend making nng::error convertible to std::error_code and/or adding a method for doing so to nng::exception.

The POSIX error codes supported by C++11 are listed here: https://en.cppreference.com/w/cpp/error/errc
The following codes have no direct mapping to a C++/POSIX equivalent.

NNG CODE	possible C++ mapping
ECLOSED
ESTATE
EUNREACHABLE, EADDRINVAL	address_not_available
ENOFILES
EREADONLY / EWRITEONLY
ECRYPTO / EPEERAUTH
ENOARG / EAMBIGUOUS / EBADTYPE
ECONNSHUT	connection_aborted
EINTERNAL
ESYSERR
ETRANERR

These codes would need to be either lossily mapped to C++ codes or implemented as an NNG-specific error_condition. It would be somewhat advantageous to assign them values that don't conflict with the standard C++ ones.

[EvanBalster]

I'm working on a pull request which adds support for <system_error> to error.h without modifying or removing any current functionality.

These are my notes as I attempt to complete the mapping to generic_error_category based on the POSIX standard and a partial mapping in NNG's nn.c.

NNG CODE	POSIX	C++ errc	Basis of mapping
ECLOSED	EBADF	bad_file_descriptor	nn.c
ESTATE	ENOTTY	inappropriate_io_control_operation	POSIX (nn.c uses EFSM)
EUNREACHABLE	EHOSTUNREACH	host_unreachable	nn.c
EADDRINVAL	EADDRNOTAVAIL	address_not_available	POSIX (nn.c uses EINVAL)
ENOFILES	EMFILE	too_many_files_open	nn.c
EREADONLY / EWRITEONLY	†EACCES	permission_denied	nn.c
ECRYPTO / EPEERAUTH	†EACCES	permission_denied	nn.c
ENOARG	†EINVAL	invalid_argument	"option requires argument"
EAMBIGUOUS	†EINVAL	invalid_argument	nn.c
EBADTYPE	†EINVAL	invalid_argument	nn.c
ECONNSHUT	†ECONNABORTED	connection_aborted	closest available
EINTERNAL	—	—	NNG-specific
ESYSERR (flag)	—	—	platform-specific
ETRANERR (flag)	—	—	transport-specific

† "Lossy" mappings; multiple NNG error codes would map to one POSIX error in default_error_condition.