change error value "fatal io error" to just "io error"

"fatal" was meant as "we need fatal for the connection, it will be dropped".
but it sounds more serious, as if something needs to be fixed.

hopefully enough for issue #39 by ArnoSen

imapserver/server.go

@@ -288,8 +288,8 @@ var commands = map[string]func(c *conn, tag, cmd string, p *parser){
 	"uid move":    (*conn).cmdUIDMove,
 }
 
-var errIO = errors.New("fatal io error")             // For read/write errors and errors that should close the connection.
-var errProtocol = errors.New("fatal protocol error") // For protocol errors for which a stack trace should be printed.
+var errIO = errors.New("io error")             // For read/write errors and errors that should close the connection.
+var errProtocol = errors.New("protocol error") // For protocol errors for which a stack trace should be printed.
 
 var sanityChecks bool
 

smtpserver/server.go

@@ -63,7 +63,7 @@ var xlog = mlog.New("smtpserver")
 
 // We use panic and recover for error handling while executing commands.
 // These errors signal the connection must be closed.
-var errIO = errors.New("fatal io error")
+var errIO = errors.New("io error")
 
 // If set, regular delivery/submit is sidestepped, email is accepted and
 // delivered to the account named mox.