C#: Add note about the class name in instantiate error

Adds a note about the requirement that a C# class name must match the
script filename in which the they are defined to the instantiate error.

modules/mono/csharp_script.cpp

@@ -2292,7 +2292,7 @@ bool CSharpScript::can_instantiate() const {
 	// For tool scripts, this will never fire if the class is not found. That's because we
 	// don't know if it's a tool script if we can't find the class to access the attributes.
 	if (extra_cond && !valid) {
-		ERR_FAIL_V_MSG(false, "Cannot instance script because the associated class could not be found. Script: '" + get_path() + "'.");
+		ERR_FAIL_V_MSG(false, "Cannot instance script because the associated class could not be found. Script: '" + get_path() + "'. Make sure the script exists and contains a class definition with a name that matches the filename of the script exactly (it's case-sensitive).");
 	}
 
 	return valid && extra_cond;