docs: public static CompletionException combineErrors( Throwable error1, Throwable error2 )
      {
          if ( error1 != null && error2 != null )
          {
              Throwable cause1 = completionExceptionCause( error1 );
              Throwable cause2 = completionExceptionCause( error2 );
              if ( cause1 != cause2 )
              {
                  cause1.addSuppressed( cause2 );
              }
              return asCompletionException( cause1 );
          }
          else if ( error1 != null )
          {
              return asCompletionException( error1 );
          }
          else if ( error2 != null )
          {
              return asCompletionException( error2 );
          }
          else
          {
              return null;
          }
      }