clean up final abort in macroexpand

shell/print.mu

@@ -298,7 +298,14 @@ fn print-pair _in: (addr cell), out: (addr stream byte), trace: (addr trace) {
     {
       compare right-addr, 0
       break-if-!=
-      abort "NULL in print!"
+      {
+        var overflow?/eax: boolean <- try-write out, " ... NULL"
+        compare overflow?, 0/false
+        break-if-=
+        error trace, "print-pair: no space for ' ... NULL'"
+        return
+      }
+      return
     }
     {
       var right-nil?/eax: boolean <- nil? right-addr

shell/sandbox.mu

@@ -613,6 +613,12 @@ fn run-sandbox _self: (addr sandbox), globals: (addr global-table) {
   rewind-keyboard-var inner-keyboard-var  # don't clear keys from before
   #
   read-and-evaluate-and-save-gap-buffer-to-globals data-ah, eval-result-ah, globals, definitions-created, trace, inner-screen-var, inner-keyboard-var
+  var error?/eax: boolean <- has-errors? trace
+  {
+    compare error?, 0/false
+    break-if-=
+    return
+  }
   # if necessary, initialize a new gap-buffer for sandbox
   {
     compare globals, 0