better handle user input in hex_string_to_hex_value

Adapt calling sites too.

When data comes from the user, it is good to check that
what we read is correct and warn the user if it's not.

common/utils/utils.h

@@ -8,9 +8,9 @@ void *calloc_or_fail(size_t size);
 void *malloc_or_fail(size_t size);
 
 // Converts an hexadecimal ASCII coded digit into its value. **
-uint8_t hex_char_to_hex_value (char c);
+int hex_char_to_hex_value (char c);
 // Converts an hexadecimal ASCII coded string into its value.**
-void hex_string_to_hex_value (uint8_t *hex_value, const char *hex_string, int size);
+int hex_string_to_hex_value (uint8_t *hex_value, const char *hex_string, int size);
 
 char *itoa(int i);