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.

src/eval.c

@@ -7999,6 +7999,7 @@ ex_echo(exarg_T *eap)
     int		needclr = TRUE;
     int		atstart = TRUE;
     char_u	numbuf[NUMBUFLEN];
+    int		did_emsg_before = did_emsg;
 
     if (eap->skip)
 	++emsg_skip;
@@ -8016,7 +8017,7 @@ ex_echo(exarg_T *eap)
 	     * 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;
 	    break;

src/message.c

@@ -636,7 +636,7 @@ emsg(char_u *s)
 	if (cause_errthrow(s, severe, &ignore) == TRUE)
 	{
 	    if (!ignore)
-		did_emsg = TRUE;
+		++did_emsg;
 	    return TRUE;
 	}
 
@@ -689,7 +689,7 @@ emsg(char_u *s)
 	    beep_flush();		/* also includes flush_buffers() */
 	else
 	    flush_buffers(FLUSH_MINIMAL);  // flush internal buffers
-	did_emsg = TRUE;		   // flag for DoOneCmd()
+	++did_emsg;			   // flag for DoOneCmd()
 #ifdef FEAT_EVAL
 	did_uncaught_emsg = TRUE;
 #endif

src/testdir/test108.ok

@@ -28,7 +28,6 @@ undefined var3 on former level:
 Error detected while processing function Foo[2]..Bar[2]..Bazz:
 line    3:
 E121: Undefined variable: var3
-E15: Invalid expression: var3
 here var3 is defined with "another var":
 another var
 
@@ -36,7 +35,6 @@ undefined var2 on former level
 Error detected while processing function Foo[2]..Bar:
 line    3:
 E121: Undefined variable: var2
-E15: Invalid expression: var2
 here var2 is defined with 10:
 10
 

src/version.c

@@ -792,6 +792,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    550,
 /**/
     549,
 /**/