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.
