Allow for error or refusal while absorbing a ProcSignalBarrier.

 Allow for error or refusal while absorbing a ProcSignalBarrier.
 
 Previously, the per-barrier-type functions tasked with absorbing
 them were expected to always succeed and never throw an error.
 However, that's a bit inconvenient. Further study has revealed that
 there are realistic cases where it might not be possible to absorb
 a ProcSignalBarrier without terminating the transaction, or even
 the whole backend. Similarly, for some barrier types, there might
 be other reasons where it's not reasonably possible to absorb the
 barrier at certain points in the code, so provide a way for a
 per-barrier-type function to reject absorbing the barrier.
 
 Unfortunately, there's still no committed code making use of this
 infrastructure; hopefully, we'll get there. :-(
 
 Patch by me, reviewed by Andres Freund and Amul Sul.
 
 Discussion: http://postgr.es/m/20200908182005.xya7wetdh3pndzim@alap3.anarazel.de
 Discussion: http://postgr.es/m/CA+Tgmob56Pk1-5aTJdVPCWFHon7me4M96ENpGe9n_R4JUjjhZA@mail.gmail.com