Evaluation failure of node expressions

[robert-david]

SHACL Core describes that evaluation of node expressions may result in an evaluation failure. However, no details of how that happens or consequences are given.

We can go a similar route to recursion and mention that processing of such failures is not defined in SHACL and is left to SHACL processor implementations.

Also, constant node expressions, which are the only node expressions mentioned in SHACL Core, should not result in an evaluation failure. However, SHACL Core still mentions this which might be confusing to readers.

[afs]

It might depend on usage:

For validation: A violation would seem appropriate -- "can not validate", "uncertain", ... -- on the principle that "constraint satisfied" should mean "known to be valid" and errors are neither yes nor no.

For Rules: the principle proposed is that inferred triples should not are caused by (unexpected) errors. Any error will mean the body does not match and no triples are inferred.

For UI: could be more like "there was an error" indicator.