vim-patch:9.0.1522: some functions give two error messages

Problem:    Some functions give two error messages.
Solution:   Do not give a second error message. (closes vim/vim#12352)

e4098457ab

It seems that tv_get_bool() is actually not exactly the same as
tv_get_number(), so change it to a function instead.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

test/old/testdir/test_expr.vim

@@ -154,6 +154,8 @@ func Test_strcharpart()
       call assert_equal('edit', "editor"[-10 : 3])
   END
   call CheckLegacyAndVim9Success(lines)
+
+  call assert_fails('echo strcharpart("", 0, 0, {})', ['E728:', 'E728:'])
 endfunc
 
 func Test_getreg_empty_list()