scpi: comment on indefinite length block data (unsupported here)

scpi: comment on indefinite length block data (unsupported here)

It's fine that the current SCPI implementation in libsigrok exclusively
supports definite length block response data. It is what most supported
devices or all of them are speaking. But the implementation of the
indefinite length case assumed that the block is empty when the response
starts with #0, and returned successful although receive data was not
taken at all.

Rephrase the test condition for successful reception. Return an error
code in the unsupported case. Emit an error message to remain aware. Let
users report when that communication pattern is seen in the field. Add
comments with references to the specs, in case the feature will receive
support later.