inccommand: Suppress error reporting when previewing commands

Closes #5912

src/nvim/ex_cmds.c

@@ -6093,7 +6093,6 @@ void ex_substitute(exarg_T *eap)
   int save_w_p_cul = curwin->w_p_cul;
   int save_w_p_cuc = curwin->w_p_cuc;
 
-  emsg_off++;                 // No error messages during command preview.
   curbuf->b_p_ul = LONG_MAX;  // make sure we can undo all changes
   curwin->w_p_cul = false;    // Disable 'cursorline'
   curwin->w_p_cuc = false;    // Disable 'cursorcolumn'
@@ -6120,6 +6119,5 @@ void ex_substitute(exarg_T *eap)
   restore_search_patterns();
   win_size_restore(&save_view);
   ga_clear(&save_view);
-  emsg_off--;
   unblock_autocmds();
 }

src/nvim/ex_getln.c

@@ -1607,7 +1607,9 @@ static int command_line_changed(CommandLineState *s)
     //       - Update the screen while the effects are in place.
     //       - Immediately undo the effects.
     State |= CMDPREVIEW;
+    emsg_silent++;  // Block error reporting as the command may be incomplete
     do_cmdline(ccline.cmdbuff, NULL, NULL, DOCMD_KEEPLINE|DOCMD_NOWAIT);
+    emsg_silent--;  // Unblock error reporting
 
     // Restore the window "view".
     curwin->w_cursor   = s->old_cursor;

test/functional/ui/inccommand_spec.lua

@@ -1201,6 +1201,40 @@ describe(":substitute, 'inccommand' with a failing expression", function()
     end
   end)
 
+  it('in the range does not error #5912', function()
+    for _, case in pairs(cases) do
+      refresh(case)
+      feed(':100s/')
+
+      screen:expect([[
+        Inc substitution on |
+        two lines           |
+                            |
+        {15:~                   }|
+        {15:~                   }|
+        {15:~                   }|
+        {15:~                   }|
+        {15:~                   }|
+        {15:~                   }|
+        :100s/^              |
+      ]])
+
+      feed('<enter>')
+      screen:expect([[
+        Inc substitution on |
+        two lines           |
+        ^                    |
+        {15:~                   }|
+        {15:~                   }|
+        {15:~                   }|
+        {15:~                   }|
+        {15:~                   }|
+        {15:~                   }|
+        {14:E16: Invalid range}  |
+      ]])
+    end
+  end)
+
 end)
 
 describe("'inccommand' and :cnoremap", function()