Fix and document return value of basic_consume

[matusvalo]

Channel.basic_consume() now has 2 return values:

• consumer_tag - when nowait == False
• promise object wen nowait == True
• ValueError() when nowait == True and user does not specify consumer_tag

This is confusing since the API is schizophrenic and returns two different values. Moreover promise object does not have any value to user of library.

I propose that consumer_tag is always returned except the case when ValueError() is raised

Also the return values are not documented.