Fix swapped parameters in ArgumentException and ArgumentNullException cstrs

The constructors of ArgumentException and ArgumentNullException are inconsistent with each other, which results in their parameters being swapped here and there.

src/Discord.Net.Core/Entities/Messages/EmbedBuilder.cs

@@ -75,7 +75,7 @@ namespace Discord
             get => _fields;
             set
             {
-                if (value == null) throw new ArgumentNullException("Cannot set an embed builder's fields collection to null", nameof(Fields));
+                if (value == null) throw new ArgumentNullException(nameof(Fields), "Cannot set an embed builder's fields collection to null");
                 if (value.Count > MaxFieldCount) throw new ArgumentException($"Field count must be less than or equal to {MaxFieldCount}.", nameof(Fields));
                 _fields = value;
             }

src/Discord.Net.Core/Utils/Preconditions.cs

@@ -45,10 +45,7 @@ namespace Discord
         }        
 
         private static ArgumentException CreateNotEmptyException(string name, string msg)
-        {
-            if (msg == null) return new ArgumentException("Argument cannot be blank", name);
-            else return new ArgumentException(name, msg);
-        }
+            => new ArgumentException(msg ?? "Argument cannot be blank.", name);
 
         //Numerics
         public static void NotEqual(sbyte obj, sbyte value, string name, string msg = null) { if (obj == value) throw CreateNotEqualException(name, msg, value); }

src/Discord.Net.Core/Utils/TokenUtils.cs

@@ -19,7 +19,7 @@ namespace Discord
         {
             // A Null or WhiteSpace token of any type is invalid.
             if (string.IsNullOrWhiteSpace(token))
-                throw new ArgumentNullException("A token cannot be null, empty, or contain only whitespace.", nameof(token));
+                throw new ArgumentNullException(nameof(token), "A token cannot be null, empty, or contain only whitespace.");
 
             switch (tokenType)
             {