Optional message to user when terminating/cancelling backend

Applying: Add infrastructure for signalling backends with payload
Using index info to reconstruct a base tree...
M	src/backend/storage/ipc/Makefile
M	src/backend/storage/ipc/ipci.c
M	src/backend/utils/init/postinit.c
Falling back to patching base and 3-way merge...
Auto-merging src/backend/utils/init/postinit.c
CONFLICT (content): Merge conflict in src/backend/utils/init/postinit.c
Auto-merging src/backend/storage/ipc/ipci.c
CONFLICT (content): Merge conflict in src/backend/storage/ipc/ipci.c
Auto-merging src/backend/storage/ipc/Makefile
CONFLICT (content): Merge conflict in src/backend/storage/ipc/Makefile
Patch failed at 0001 Add infrastructure for signalling backends with payload

error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
hint: When you have resolved this problem, run "git am --continue".
hint: If you prefer to skip this patch, run "git am --skip" instead.
hint: To restore the original branch and stop patching, run "git am --abort".
hint: Disable this message with "git config advice.mergeConflict false"