check and log errors more often in deferred cleanup calls, and log remote-induced errors at lower priority

We normally check errors for all operations. But for some cleanup calls, eg
"defer file.Close()", we didn't. Now we also check and log most of those.
Partially because those errors can point to some mishandling or unexpected code
paths (eg file unexpected already closed). And in part to make it easier to use
"errcheck" to find the real missing error checks, there is too much noise now.

The log.Check function can now be used unconditionally for checking and logging
about errors. It adjusts the log level if the error is caused by a network
connection being closed, or a context is canceled or its deadline reached, or a
socket deadline is reached.

store/account.go

@@ -1119,9 +1119,11 @@ func OpenAccountDB(log mlog.Log, accountDir, accountName string) (a *Account, re
 
 	defer func() {
 		if rerr != nil {
-			db.Close()
+			err := db.Close()
+			log.Check(err, "closing database file after error")
 			if isNew {
-				os.Remove(dbpath)
+				err := os.Remove(dbpath)
+				log.Check(err, "removing new database file after error")
 			}
 		}
 	}()
@@ -1820,17 +1822,26 @@ func (a *Account) CheckConsistency() error {
 		conf, _ := a.Conf()
 		if conf.JunkFilter != nil {
 			random := make([]byte, 16)
-			cryptorand.Read(random)
+			if _, err := cryptorand.Read(random); err != nil {
+				return fmt.Errorf("reading random: %v", err)
+			}
 			dbpath := filepath.Join(mox.DataDirPath("tmp"), fmt.Sprintf("junkfilter-check-%x.db", random))
 			bloompath := filepath.Join(mox.DataDirPath("tmp"), fmt.Sprintf("junkfilter-check-%x.bloom", random))
 			os.MkdirAll(filepath.Dir(dbpath), 0700)
-			defer os.Remove(dbpath)
-			defer os.Remove(bloompath)
+			defer func() {
+				err := os.Remove(bloompath)
+				log.Check(err, "removing temp bloom file")
+				err = os.Remove(dbpath)
+				log.Check(err, "removing temp junk filter database file")
+			}()
 			jf, err = junk.NewFilter(ctx, log, conf.JunkFilter.Params, dbpath, bloompath)
 			if err != nil {
 				return fmt.Errorf("new junk filter: %v", err)
 			}
-			defer jf.Close()
+			defer func() {
+				err := jf.Close()
+				log.Check(err, "closing junk filter")
+			}()
 		}
 		var ntrained int
 
@@ -1987,7 +1998,10 @@ func (a *Account) CheckConsistency() error {
 			if err != nil {
 				return fmt.Errorf("open account junk filter: %v", err)
 			}
-			defer ajf.Close()
+			defer func() {
+				err := ajf.Close()
+				log.Check(err, "closing junk filter")
+			}()
 			wordsGot, err := load(ajf)
 			if err != nil {
 				return fmt.Errorf("read account junk filter: %v", err)

store/threads.go

@@ -423,7 +423,10 @@ func (a *Account) AssignThreads(ctx context.Context, log mlog.Log, txOpt *bstore
 					buf := headerbuf[:int(size)]
 					err := func() error {
 						mr := a.MessageReader(m)
-						defer mr.Close()
+						defer func() {
+							err := mr.Close()
+							log.Check(err, "closing message reader")
+						}()
 
 						// ReadAt returns whole buffer or error. Single read should be fast.
 						n, err := mr.ReadAt(buf, partialPart.HeaderOffset)