Clarify why path-computation-error-reason has not been defined in an IANA-maintained model

[italobusi]

Offload path-computation-error-reason to an IANA-maintained model. The current design is suboptimal. Please refer to the 84047bis on these matters.

[italobusi]

The document is defining additional identities which are applicable to other contexts than PCE and in some cases they are in hierarchical relationship with the identities which applies to PCE:

path-computation-error-reason (base identity)
   path-computation-error-no-topology (TEAS-specific)
   path-computation-error-no-dependent-server (TEAS-specific)
      path-computation-error-child-pce-unresponsive (IANA)
      base path-computation-error-no-dependent-server (IANA)
   path-computation-error-pce-unavailable (IANA)

The IANA registry is flat so it is not possible to record in the registry the need for deriving the path-computation-error-child-pce-unresponsive and the path-computation-error-no-dependent-server from the path-computation-error-no-dependent-server instead of from the path-computation-error-reason like other identities (e.g., path-computation-error-pce-unavailable)