Never throw exception in fireplace#op_available

plugin/fireplace.vim

@@ -556,10 +556,13 @@ function! fireplace#message(payload, ...) abort
 endfunction
 
 function! fireplace#op_available(op) abort
+  try
     let client = fireplace#platform()
     if has_key(client, 'connection')
       return client.connection.has_op(a:op)
     endif
+  catch /^Fireplace: :Connect to a REPL/
+  endtry
 endfunction
 
 function! fireplace#findresource(resource, ...) abort