Introduce vacuum errcontext to display additional information.

 Introduce vacuum errcontext to display additional information.
 
 The additional information displayed will be block number for error
 occurring while processing heap and index name for error occurring
 while processing the index.
 
 This will help us in diagnosing the problems that occur during a vacuum.
 For ex. due to corruption (either caused by bad hardware or by some bug)
 if we get some error while vacuuming, it can help us identify the block
 in heap and or additional index information.
 
 It sets up an error context callback to display additional information
 with the error.  During different phases of vacuum (heap scan, heap
 vacuum, index vacuum, index clean up, heap truncate), we update the error
 context callback to display appropriate information.  We can extend it to
 a bit more granular level like adding the phases for FSM operations or for
 prefetching the blocks while truncating. However, I felt that it requires
 adding many more error callback function calls and can make the code a bit
 complex, so left those for now.
 
 Author: Justin Pryzby, with few changes by Amit Kapila
 Reviewed-by: Alvaro Herrera, Amit Kapila, Andres Freund, Michael Paquier
 and Sawada Masahiko
 Discussion: https://www.postgresql.org/message-id/20191120210600.GC30362@telsasoft.com