Don't log unexpected exits

Before this change, if we crashed inside either of these main loops, we
would still log messages about exiting the loops. Even if the reason for
exiting was a panic().

With this change in place, we only log expected exits. The idea is that
this should make it easier to understand where the crash was.

m/pager.go

@@ -271,7 +271,6 @@ func (p *Pager) handleScrolledDown() {
 // StartPaging brings up the pager on screen
 func (p *Pager) StartPaging(screen twin.Screen, chromaStyle *chroma.Style, chromaFormatter *chroma.Formatter) {
 	log.Info("Pager starting")
-	defer log.Info("Pager done")
 
 	defer func() {
 		if p.reader.err != nil {

twin/screen.go

@@ -387,15 +387,15 @@ func (screen *UnixScreen) ShowCursorAt(column int, row int) {
 }
 
 func (screen *UnixScreen) mainLoop() {
-	defer log.Info("Twin screen main loop done")
-
 	// "1400" comes from me trying fling scroll operations on my MacBook
 	// trackpad and looking at the high watermark (logged below).
 	//
 	// The highest I saw when I tried this was 700 something. 1400 is twice
 	// that, so 1400 should be good.
 	buffer := make([]byte, 1400)
 
+	log.Info("Entering Twin main loop...")
+
 	maxBytesRead := 0
 	expectingTerminalBackgroundColor := true
 	for {