Make SSL error code more generic

It's undesirable to have users of the SSL layer check for an error code
specific to a lower-level layer, both out of general layering principles, and
also because if we later make another crypto module gain resume capabilities,
we would need to change the contract again (checking for a new module-specific
error code).

library/ssl_tls.c

@@ -4683,7 +4683,7 @@ crt_verify:
 
 #if defined(MBEDTLS_SSL__ECP_RESTARTABLE)
         if( ret == MBEDTLS_ERR_ECP_IN_PROGRESS )
-            return( ret );
+            return( MBEDTLS_ERR_SSL_CRYPTO_IN_PROGRESS );
 #endif
 
         /*