Enhance error handeling.

- New API functions: nfc_strerror(), nfc_strerror_r() and nfc_perror();
  - Drivers now have a reference to chips callback methods;
  - Rename -pn53x_err2string to pn53x_strerror and add it to pn53x_callbacks_list.

include/nfc/nfc-types.h

@@ -93,6 +93,15 @@ typedef struct {
   uint32_t uiBusIndex;
 } nfc_device_desc_t;
 
+/**
+ * @struct chip_callbacks
+ * @brief Functions for chip specific functions.
+ */
+struct chip_callbacks {
+    /** Error lookup */
+    const char* (*strerror) (const nfc_device_t *pnd);
+};
+
 /**
  * @struct driver_callbacks
  * @brief Generic structure to handle NFC device functions.
@@ -100,6 +109,8 @@ typedef struct {
 struct driver_callbacks {
   /** Driver name */
   const char* acDriver;
+  /** Chip specific callback functions */
+  const struct chip_callbacks *pcc;
   /** Pick devices callback */
   nfc_device_desc_t *(*pick_device)(void);
   /** List devices callback */