Introduce GIT_EAUTH

Introduce this error code to signal an authentication failure.

include/git2/errors.h

@@ -41,6 +41,7 @@ typedef enum {
 	GIT_EMERGECONFLICT  = -13,	/**< Merge conflicts prevented operation */
 	GIT_ELOCKED         = -14,	/**< Lock file prevented operation */
 	GIT_EMODIFIED       = -15,	/**< Reference value does not match expected */
+	GIT_EAUTH           = -16,      /**< Authentication error */
 
 	GIT_PASSTHROUGH     = -30,	/**< Internal only */
 	GIT_ITEROVER        = -31,	/**< Signals end of iteration with iterator */