Platform had it's own scheme for reporting errors: NMPlatformError.
Before, NMPlatformError indicated success via zero, negative integer
values are numbers from <errno.h>, and positive integer values are
platform specific codes. This changes now according to nm-error:
success is still zero. Negative values indicate a failure, where the
numeric value is either from <errno.h> or one of our error codes.
The meaning of positive values depends on the functions. Most functions
can only report an error reason (negative) and success (zero). For such
functions, positive values should never be returned (but the caller
should anticipate them).
For some functions, positive values could mean additional information
(but still success). That depends.
This is also what systemd does, except that systemd only returns
(negative) integers from <errno.h>, while we merge our own error codes
into the range of <errno.h>.
The advantage is to get rid of one way how to signal errors. The other
advantage is, that these error codes are compatible with all other
nm-errno values. For example, previously negative values indicated error
codes from <errno.h>, but it did not entail error codes from netlink.
This will be our extension on top of <errno.h>.
We want to use (integer) error numbers, that can both
contain native errors from <errno.h> and our own defines,
both merge in one domain. That is, we will reserve a small
range of integers for our own defines (that hopefully won't
clash with errors from <errno.h>).
We can use this at places where GError is too cumbersome to use.
The advantage is, that our error numbers extend <errno.h> and can
be mixed.
This is what "src/platform/nm-netlink.h" already does with nl_errno(). Next,
the netlink errors from there will be merged into "nm-errno.h".
Also, platform has NMPlatformError, which are a distinct set of error
numbers. But these work differently in the sense that negative values
represent codes from <errno.h> and positive numbers are our own platform
specific defines. NMPlatformError will also be merged into "nm-errno.h".
"nm-errno.h" will unify the error handling of platform and netlink,
making it more similar to what we are used to from systemd, and give
room to extend it for our own purpose.
