Add error string for failure code in `mbedtls_dhm_set_group`
diff --git a/library/error.c b/library/error.c
index db42381..6fa0c21 100644
--- a/library/error.c
+++ b/library/error.c
@@ -206,6 +206,8 @@
             mbedtls_snprintf( buf, buflen, "DHM - Allocation of memory failed" );
         if( use_ret == -(MBEDTLS_ERR_DHM_FILE_IO_ERROR) )
             mbedtls_snprintf( buf, buflen, "DHM - Read/write of file failed" );
+        if( use_ret == -(MBEDTLS_ERR_DHM_SET_GROUP_FAILED) )
+            mbedtls_snprintf( buf, buflen, "DHM - Setting modulus and generator failed" );
 #endif /* MBEDTLS_DHM_C */
 
 #if defined(MBEDTLS_ECP_C)