patch 8.2.1840: Vim9: error message is not clear about compilation error

Problem:    Vim9: error message is not clear about compilation error.
Solution:   Say "compiling" instead of "processing".
diff --git a/src/message.c b/src/message.c
index 059835a..c527952 100644
--- a/src/message.c
+++ b/src/message.c
@@ -467,7 +467,12 @@
 	if (sname == NULL)
 	    sname = SOURCING_NAME;
 
-	p = (char_u *)_("Error detected while processing %s:");
+#ifdef FEAT_EVAL
+	if (estack_compiling)
+	    p = (char_u *)_("Error detected while compiling %s:");
+	else
+#endif
+	    p = (char_u *)_("Error detected while processing %s:");
 	Buf = alloc(STRLEN(sname) + STRLEN(p));
 	if (Buf != NULL)
 	    sprintf((char *)Buf, (char *)p, sname);