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/userfunc.c b/src/userfunc.c
index 79e711c..be1eaa9 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -5028,7 +5028,7 @@
 		if ((cstack->cs_rettv[idx] = alloc_tv()) != NULL)
 		    *(typval_T *)cstack->cs_rettv[idx] = *(typval_T *)rettv;
 		else
-		    emsg(_(e_outofmem));
+		    emsg(_(e_out_of_memory));
 	    }
 	    else
 		cstack->cs_rettv[idx] = NULL;