Most NMDevice types defined their own error domain but then never used
it. A few did use their errors, but some of those errors are redundant
with NMDeviceError, and others can be added to it.
Merge libnm's NMDeviceError and the daemon's NMDeviceError into a
single enum (in nm-errors.h). Register the domain with D-Bus, and add
a test that the client side decodes it correctly.
The daemon's NM_DEVICE_ERROR_CONNECTION_INVALID gets absorbed into
libnm's NM_DEVICE_ERROR_INVALID_CONNECTION, and
NM_DEVICE_ERROR_UNSUPPORTED_DEVICE_TYPE gets dropped, since it was
only returned from one place, which is now using
NM_DEVICE_ERROR_FAILED, since (a) it ought to be a "can't happen", and
(b) the only caller of that function just logs error->message and then
frees the error without ever looking at the code.
Register NMConnectionError with D-Bus on both sides, so that, eg,
connection validation failures in the daemon will translate to the
correct error codes in the client.
Add nm-errors.[ch], and move libnm-core's two error domains
(NMConnectionError and NMCryptoError) there.
NMCryptoError wasn't previously visible, but it can be returned from
some public API, so it should be.
