vim-patch:8.1.0550: expression evaluation may repeat an error message

Problem:    Expression evaluation may repeat an error message. (Jason
            Franklin)
Solution:   Increment did_emsg and check for the value when giving an error
            for the echo command.
76a6345433

src/nvim/eval.c

@@ -19538,6 +19538,7 @@ void ex_echo(exarg_T *eap)
   typval_T rettv;
   bool needclr = true;
   bool atstart = true;
+  const int did_emsg_before = did_emsg;
 
   if (eap->skip)
     ++emsg_skip;
@@ -19552,7 +19553,7 @@ void ex_echo(exarg_T *eap)
         // Report the invalid expression unless the expression evaluation
         // has been cancelled due to an aborting error, an interrupt, or an
         // exception.
-        if (!aborting()) {
+        if (!aborting() && did_emsg == did_emsg_before) {
           EMSG2(_(e_invexpr2), p);
         }
         need_clr_eos = false;