Add common feature unavailable error

Add a common error for the feature unavailable, in the
platform module.
diff --git a/library/error.c b/library/error.c
index 27692db..eabee9e 100644
--- a/library/error.c
+++ b/library/error.c
@@ -834,6 +834,8 @@
 #if defined(MBEDTLS_PLATFORM_C)
     if( use_ret == -(MBEDTLS_ERR_PLATFORM_HW_ACCEL_FAILED) )
         mbedtls_snprintf( buf, buflen, "PLATFORM - Hardware accelerator failed" );
+    if( use_ret == -(MBEDTLS_ERR_PLATFORM_FEATURE_UNSUPPORTED) )
+        mbedtls_snprintf( buf, buflen, "PLATFORM - The requested feature is not supported by the platform" );
 #endif /* MBEDTLS_PLATFORM_C */
 
 #if defined(MBEDTLS_POLY1305_C)