Fix GetVectorById for Faiss_HNSW

[liliu-z]

          `GetVectorById` need to return an error code if there is no raw data available (If the data type is FP16, then the raw data need to be FP16).

So we need a raw data existence check there.

Originally posted by @liliu-z in #746 (comment)

[liliu-z]

/assign
/assign @alexanderguzhva

[alexanderguzhva]

@alexanderguzhva Isn't it handled by ::HasRawData() method?

[liliu-z]

@alexanderguzhva Isn't it handled by ::HasRawData() method?

Good question. Sorry the API definition isn't clear. The assumption is that HasRawData & GetVectorById are two individual function, meaning ppl are allow to call GetVectorById without calling HasRawData before. Although we hope users to call them in order, there are no hard limit to guarantee this.