Prepare for future removal of support for unbalanced transactions

Don't worry, that won't happen for at least months. First we'll just log
warnings for a while.

plugins/log

@@ -2,9 +2,11 @@
 
 my $filename = ".revbank.log";
 
-sub _log {
+sub _log($tag, @message) {
+    @message = ("") if not @message;
+
     open my $fh, '>>', $filename or warn "$filename: $!";
-    print $fh now(), " ", @_, "\n";
+    print $fh map(s/^/now() . " $tag "/rgme, @message), "\n";
     close $fh or warn "$filename: $!";
 }
 
@@ -18,28 +20,37 @@ sub hook_prompt($class, $cart, $prompt, @) {
 
 sub hook_input($class, $cart, $input, $split_input, @) {
     $input //= "(UNDEF)";
-    _log("PROMPT $buffer{prompt} >> $input");
+    $input = "(EMPTY)" if not length $input;
+    _log(PROMPT => "$buffer{prompt} >> $input");
 }
 
 sub hook_reject($class, $plugin, $reason, $abort, @) {
-    _log("REJECT [$plugin] $reason");
+    _log(REJECT => "[$plugin] $reason");
 }
 
 sub hook_retry($class, $plugin, $reason, $abort, @) {
-    _log("RETRY [$plugin] $reason");
+    _log(RETRY => "[$plugin] $reason");
 }
 
 sub hook_user_created($class, $username, @) {
-    _log("NEWUSER $username");
+    _log(NEWUSER => "$username");
 }
 
 sub hook_user_balance($class, $user, $old, $delta, $new, $transaction_id, @) {
     my $lost = $delta < 0 ? "lost" : "got";
     $delta = $delta->abs;
     $_ = $_->string("+") for $old, $new;
-    _log("BALANCE $transaction_id $user had $old, $lost $delta, now has $new");
+    _log(BALANCE => "$transaction_id $user had $old, $lost $delta, now has $new");
 }
 
 sub hook_checkout($class, $cart, $username, $transaction_id, @) {
-    _log("CHECKOUT $transaction_id $_") for map $_->as_loggable, $cart->entries;
+    _log(CHECKOUT => "$transaction_id $_") for map $_->as_loggable, $cart->entries;
+}
+
+sub hook_log_warning($class, $message, @) {
+    _log(WARNING => $message);
+}
+
+sub hook_log_error($class, $message, @) {
+    _log(ERROR => $message);
 }