[req] raise KeyError instead of AssertionError in req.find()

[req] use format() instead of % substitution For readability, avoid escaping using %% and use format() instead.

[test] make assertions on exception message

[req] clearer exception message Display what attribute trigger the NotImplementedError

[cleanup] Drop useless fake function actually not necessary.

[cleanup] Use plain unittest main in unittest_rqlrewrite

[cleanup] Fix flake8 error unittest_rqlrewrite hard one!

[rqlrewrite] Enhance detection of need for Exists node We actually want one only if this is not a (`And` / `Or`) binary tree of `Not` or `Exists` nodes, so write a recursive function to tell so. Related to #17074119

[rqlrewrite] Fix rewrite on ambiguities introduced by NOT relation or "is IN" type restriction When some inserted RQL snippet generate more solutions than the original RQL, the rewriter attempt to duplicate the snippet for each newly introduced solution. There are though some cases where we do not want this behaviour in case of ambiguities introduced by: * NOT(X relation Y) expression, since it won't be equivalent to NOT(X relation Y1, Y1 is Type1) OR NOT(X relation Y2, Y2 is Type2) ; * EXISTS(X relation Y, Y is IN (Type1, Type2) expression, since it's not actually necessary to split an explicitly introduced ambiguity (and it crash if we attempt to do so, so...). In test, we've to modify the `rewrite()` function because in the newly introduced test we need the same constraint to be applied to two variables in the original query, and this was not supported before. Notice the generated RQL in test is still *NOT CORRECT* "(EXISTS(NOT EXISTS() OR EXISTS(...))", or at least isn't optimal. This will be fixed in a forthcoming changeset. Related to #17074119

[dataimport] Add explanation about why external entities can't be inserted By default after the import processed the importer indicates which external entities can't be inserted because they are missing dependency data (other entities, used in inlined or mandatory relations). It usually helps there to find out which extids / relations are missing, so add this to the log.