Document TS.ADD error modes

[miohtama]

Currently TS.ADD does not cover what could go wrong.

E.g. when trying to reuse the same timestamp you get

TSDB: Error at upsert, update is not supported in BLOCK mode

The error itself is very technical, could probably converted to some more user friendly error like "timestamp already exist".

[miohtama]

Another error is invalid values e.g. RedisTimeSeries does not take NaN as a value.