refactor: using a different error number for 'mousescroll'

Because E548 is linked to 'guicursor' in help.

runtime/doc/options.txt

@@ -4254,7 +4254,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	Warning: Setting this option can make pending mappings to be aborted
 	when the mouse is moved.
 
-						*'mousescroll'*
+						*'mousescroll'* *E5080*
 'mousescroll'		string	(default "ver:3,hor:6")
 			global
 	This option controls the number of lines / columns to scroll by when

src/nvim/optionstr.c

@@ -514,7 +514,7 @@ const char *did_set_mousescroll(optset_T *args FUNC_ATTR_UNUSED)
     // Verify that only digits follow the colon.
     for (size_t i = 4; i < length; i++) {
       if (!ascii_isdigit(string[i])) {
-        return N_("E548: digit expected");
+        return N_("E5080: Digit expected");
       }
     }
 

test/functional/options/mousescroll_spec.lua

@@ -20,7 +20,7 @@ end
 
 describe("'mousescroll'", function()
   local invalid_arg = 'Vim(set):E474: Invalid argument: mousescroll='
-  local digit_expected = 'Vim(set):E548: digit expected: mousescroll='
+  local digit_expected = 'Vim(set):E5080: Digit expected: mousescroll='
 
   local function should_fail(val, errorstr)
     eq(errorstr..val, exc_exec('set mousescroll='..val))