Each setting type was defining its own error type, but most of them
had exactly the same three errors ("unknown", "missing property", and
"invalid property"), and none of the other values was of much use
programmatically anyway.

So, this commit merges NMSettingError, NMSettingAdslError, etc, all
into NMConnectionError. (The reason for merging into NMConnectionError
rather than NMSettingError is that we also already have
"NMSettingsError", for errors related to the settings service, so
"NMConnectionError" is a less-confusable name for settings/connection
errors than "NMSettingError".)

Also, make sure that all of the affected error messages are localized,
and (where appropriate) prefix them with the relevant property name.

Renamed error codes:

NM_SETTING_ERROR_PROPERTY_NOT_FOUND -> NM_CONNECTION_ERROR_PROPERTY_NOT_FOUND
NM_SETTING_ERROR_PROPERTY_NOT_SECRET -> NM_CONNECTION_ERROR_PROPERTY_NOT_SECRET

Remapped error codes:

NM_SETTING_*_ERROR_MISSING_PROPERTY -> NM_CONNECTION_ERROR_MISSING_PROPERTY
NM_SETTING_*_ERROR_INVALID_PROPERTY -> NM_CONNECTION_ERROR_INVALID_PROPERTY
NM_SETTING_ERROR_PROPERTY_TYPE_MISMATCH -> NM_CONNECTION_ERROR_INVALID_PROPERTY
NM_SETTING_BLUETOOTH_ERROR_TYPE_SETTING_NOT_FOUND -> NM_CONNECTION_ERROR_INVALID_SETTING
NM_SETTING_BOND_ERROR_INVALID_OPTION -> NM_CONNECTION_ERROR_INVALID_PROPERTY
NM_SETTING_BOND_ERROR_MISSING_OPTION -> NM_CONNECTION_ERROR_MISSING_PROPERTY
NM_SETTING_CONNECTION_ERROR_TYPE_SETTING_NOT_FOUND -> NM_CONNECTION_ERROR_MISSING_SETTING
NM_SETTING_CONNECTION_ERROR_SLAVE_SETTING_NOT_FOUND -> NM_CONNECTION_ERROR_MISSING_SETTING
NM_SETTING_IP4_CONFIG_ERROR_NOT_ALLOWED_FOR_METHOD -> NM_CONNECTION_ERROR_INVALID_PROPERTY
NM_SETTING_IP6_CONFIG_ERROR_NOT_ALLOWED_FOR_METHOD -> NM_CONNECTION_ERROR_INVALID_PROPERTY
NM_SETTING_VLAN_ERROR_INVALID_PARENT -> NM_CONNECTION_ERROR_INVALID_PROPERTY
NM_SETTING_WIRELESS_SECURITY_ERROR_MISSING_802_1X_SETTING -> NM_CONNECTION_ERROR_MISSING_SETTING
NM_SETTING_WIRELESS_SECURITY_ERROR_LEAP_REQUIRES_802_1X -> NM_CONNECTION_ERROR_INVALID_PROPERTY
NM_SETTING_WIRELESS_SECURITY_ERROR_LEAP_REQUIRES_USERNAME -> NM_CONNECTION_ERROR_MISSING_PROPERTY
NM_SETTING_WIRELESS_SECURITY_ERROR_SHARED_KEY_REQUIRES_WEP -> NM_CONNECTION_ERROR_INVALID_PROPERTY
NM_SETTING_WIRELESS_ERROR_CHANNEL_REQUIRES_BAND -> NM_CONNECTION_ERROR_MISSING_PROPERTY

Dropped error codes (were previously defined but unused):

NM_SETTING_CDMA_ERROR_MISSING_SERIAL_SETTING
NM_SETTING_CONNECTION_ERROR_IP_CONFIG_NOT_ALLOWED
NM_SETTING_GSM_ERROR_MISSING_SERIAL_SETTING
NM_SETTING_PPP_ERROR_REQUIRE_MPPE_NOT_ALLOWED
NM_SETTING_PPPOE_ERROR_MISSING_PPP_SETTING
NM_SETTING_SERIAL_ERROR_MISSING_PPP_SETTING
NM_SETTING_WIRELESS_ERROR_MISSING_SECURITY_SETTING