Move the definition of NMManagerError to nm-errors, register it with
D-Bus, and verify in the tests that it maps correctly.

NM_MANAGER_ERROR_INTERNAL gets renamed to NM_MANAGER_ERROR_FAILED for
consistency. NM_MANAGER_ERROR_UNMANAGED_DEVICE is dropped since that
name doesn't really describe the one place it was previously used in.
NM_MANAGER_ERROR_SYSTEM_CONNECTION is dropped because it was't being
used. NM_MANAGER_ERROR_UNSUPPORTED_CONNECTION_TYPE is dropped because
it can be replaced with an NM_CONNECTION_ERROR.
NM_MANAGER_ERROR_AUTOCONNECT_NOT_ALLOWED is turned into the more
generic NM_MANAGER_ERROR_CONNECTION_NOT_AVAILABLE.

Also, remove the <tp:possible-errors> sections from nm-manager.xml,
since they were completely out of date.