Found another one: "invalid_parameter_exception":
However, this exception still does not provide enough information in its message:
SOAP Error: SoapFault exception: [Receiver] Invalid parameter value detected, execution can not continue.
The really useful information appears to be in the "debuginfo" property.
[errorcode] => invalidparameter
[module] => debug
[debuginfo] => Username already exists: wsuser1
[message:protected] => Invalid parameter value detected, execution can not continue.
So perhaps "debuginfo" needs to be part of the message returned to the caller?