Using your example with a few changes, it appears to be working as expected. @serror is returning the correct error that the registry key does not exist (at least on my machine).

 Code:
? @kix
$ffx = DelTree("HKEY_LOCAL_MACHINE\Software\Mozilla\Mozilla Firefox abc")
? @serror


Produces
 Quote:

4.60
The system cannot find the file specified.

4.53
The system cannot find the file specified.