Tweak pg_promote() to report failures on kill() or postmaster failures

Tweak pg_promote() to report failures on kill() or postmaster failures

Since its introduction in 10074651e335, pg_promote() has been returning
a false status in three cases:
- SIGUSR1 not sent to the postmaster process.
- Postmaster death during standby promotion.
- Standby not promoted within the specified wait time.

An application calling this function will have a hard time understanding
what a false state returned actually means.

Per discussion, this switches the two first states to fail rather than
return a "false" status, making the second case more consistent with the
existing CHECK_FOR_INTERRUPTS in the wait loop.  False is only returned
when the promotion is not completed within the specified time (60s by
default).

Author: Ashutosh Sharma
Reviewed-by: Fujii Masao, Laurenz Albe, Michael Paquier
Discussion: https://postgr.es/m/CAE9k0P=QTrwptL0t4J0fuBRDDjgsT-0PVKd-ikd96i1hyL7Bcg@mail.gmail.com