patch 8.2.3190: error messages are spread out

Problem:    Error messages are spread out.
Solution:   Move error messages to errors.h and give them a clear name.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 0273f89..1efefb1 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -919,7 +919,7 @@
 	    next_cmdline = vim_strsave(next_cmdline);
 	    if (next_cmdline == NULL)
 	    {
-		emsg(_(e_outofmem));
+		emsg(_(e_out_of_memory));
 		retval = FAIL;
 		break;
 	    }
@@ -6959,7 +6959,7 @@
     static void
 ex_nogui(exarg_T *eap)
 {
-    eap->errmsg = _(e_nogvim);
+    eap->errmsg = _(e_gui_cannot_be_used_not_enabled_at_compile_time);
 }
 #endif