fmsg: implement suspend in writer

This removes the requirement to call fmsg.Exit on every exit path, and enables direct use of the "log" package. However, fmsg.BeforeExit is still encouraged when possible to catch exit on suspended output.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/system/op.go

@@ -3,6 +3,7 @@ package system
 import (
 	"context"
 	"errors"
+	"log"
 	"os"
 	"sync"
 
@@ -105,9 +106,9 @@ func (sys *I) Commit(ctx context.Context) error {
 		// sp is set to nil when all ops are applied
 		if sp != nil {
 			// rollback partial commit
-			fmsg.VPrintf("commit faulted after %d ops, rolling back partial commit", len(sp.ops))
+			fmsg.Verbosef("commit faulted after %d ops, rolling back partial commit", len(sp.ops))
 			if err := sp.Revert(&Criteria{nil}); err != nil {
-				fmsg.Println("errors returned reverting partial commit:", err)
+				log.Println("errors returned reverting partial commit:", err)
 			}
 		}
 	}()