patch 9.0.1722: wrong error messages when passing wrong types to count()

Problem: wrong error messages when passing wrong types to count()
Solution: fix it

This fixes two problems:
1. When passing wrong type to {ic} argument of count(), two error
   messages are given, the second of which is misleading.
2. When passing wrong type to {comp} argument of count(), the error
   message doesn't mention that {comp} may be a String.

closes: #12825

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: zeertzjq <zeertzjq@outlook.com>
diff --git a/src/errors.h b/src/errors.h
index 6ef4700..1c1b146 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -1802,7 +1802,8 @@
 	INIT(= N_("E704: Funcref variable name must start with a capital: %s"));
 EXTERN char e_variable_name_conflicts_with_existing_function_str[]
 	INIT(= N_("E705: Variable name conflicts with existing function: %s"));
-// E706 unused
+EXTERN char e_argument_of_str_must_be_list_string_or_dictionary[]
+	INIT(= N_("E706: Argument of %s must be a List, String or Dictionary"));
 EXTERN char e_function_name_conflicts_with_variable_str[]
 	INIT(= N_("E707: Function name conflicts with variable: %s"));
 EXTERN char e_slice_must_come_last[]