UI hangs when low gas price is set for the user transaction

[akolotov]

During the testing it was found that even if the error like on the screenshot below appears, there is no notification to the user that web3 stops waiting for the transaction to be included to a block.

This could confuse the user - it will never know that he needs to re-send the transaction.

This issue most often happens when a low gas price was set for the transaction, that's why under #83 it was decided to use the gas price marked as fast in the response of the gas price oracle. But user can re-define lower price in the wallet window so the transaction will hangs anyway.

It makes sense to catch this error and display the user an alert that the transaction need a special treatment.