Add HW_FAILED error codes for message digest modules

New error codes to report failures from alternative implementations of
MD2, MD4, MD5, RIPEMD160, SHA-1, SHA-256, SHA-512.
diff --git a/include/mbedtls/sha512.h b/include/mbedtls/sha512.h
index 7e2fcc5..395f8bb 100644
--- a/include/mbedtls/sha512.h
+++ b/include/mbedtls/sha512.h
@@ -32,6 +32,8 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#define MBEDTLS_ERR_SHA512_HW_ACCEL_FAILED                -0x0039  /**< SHA-512 hardware accelerator failed */
+
 #if ( defined(__ARMCC_VERSION) || defined(_MSC_VER) ) && \
     !defined(inline) && !defined(__cplusplus)
 #define inline __inline