SR_ERR_IO: new error code

Add new error code which can be used to notify the user about
general input/output errors.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

include/libsigrok/libsigrok.h

@@ -76,6 +76,7 @@ enum sr_error_code {
 	SR_ERR_TIMEOUT       = -8, /**< A timeout occurred. */
 	SR_ERR_CHANNEL_GROUP = -9, /**< A channel group must be specified. */
 	SR_ERR_DATA          =-10, /**< Data is invalid.  */
+	SR_ERR_IO            =-11, /**< Input/output error. */
 
 	/*
 	 * Note: When adding entries here, don't forget to also update the

src/error.c

@@ -82,6 +82,8 @@ SR_API const char *sr_strerror(int error_code)
 		return "no channel group specified";
 	case SR_ERR_DATA:
 		return "data is invalid";
+	case SR_ERR_IO:
+		return "input/output error";
 	default:
 		return "unknown error";
 	}