Align "wrong number of arguments" messages

Make "N for M" into the form "given N, expected M".

As I worked, I noticed that the `argnum_error()` function had a part to include the method name in the message.
I think this part is no longer needed by https://github.com/mruby/mruby/pull/5394.

  - Before this patch

    ```console
    % bin/mruby -e '[1, 2, 3].each 0'
    trace (most recent call last):
            [1] -e:1
    -e:1:in each: 'each': wrong number of arguments (1 for 0) (ArgumentError)
    ```

  - After this patch

    ```console
    % bin/mruby -e '[1, 2, 3].each 0'
    trace (most recent call last):
            [1] -e:1
    -e:1:in each: wrong number of arguments (given 1, expected 0) (ArgumentError)
    ```

1 files changed, 1 insertions, 1 deletions

diff --git a/mrbgems/mruby-string-ext/mrblib/string.rb b/mrbgems/mruby-string-ext/mrblib/string.rb
index d485c51ef..d8fd120a0 100644
--- a/mrbgems/mruby-string-ext/mrblib/string.rb
+++ b/mrbgems/mruby-string-ext/mrblib/string.rb
@@ -151,7 +151,7 @@ class String
   #
   def slice!(arg1, arg2=nil)
     raise FrozenError, "can't modify frozen String" if frozen?
-    raise "wrong number of arguments (for 1..2)" if arg1.nil? && arg2.nil?
+    raise ArgumentError, "wrong number of arguments (expected 1..2)" if arg1.nil? && arg2.nil?
 
     if !arg1.nil? && !arg2.nil?
       idx = arg1