`union` of `SHOW EDGES` and `SHOW TAGS` will crash graphd

[wey-gu]

This crashed graphd now.

SHOW EDGES
  union
SHOW TAGS

[jyf111]

The SHOW xxx operation returns a default iterator, but UNION/INTERSECT/MINUS assumes the iterator is sequential and does not have proper error handling, which leads to crash.

https://github.com/vesoft-inc/nebula/blob/38b3e9d88e527b53fe9677013777957af0500088/src/graph/executor/maintain/EdgeExecutor.cpp#L109-L112

https://github.com/vesoft-inc/nebula/blob/38b3e9d88e527b53fe9677013777957af0500088/src/graph/executor/query/UnionExecutor.cpp#L22

Similarly, SHOW EDGES | LIMIT 1 also crashes graphd.

Should we add more error handling or just support these operations? I guess the original intent of the default iterator does not allow these operations, so they should be disabled.

What is your expectation? @wey-gu