Imitate transfer failures

[rishiloyola]

capture outcome of the transfers:

• add a new attribute to TransferRequest struct Error error
• assign error to TransferRequest if it occurs at https://github.com/vkuznet/transfer2go/blob/master/core/request.go#L177 https://github.com/vkuznet/transfer2go/blob/master/core/request.go#L199
• check an error of job.TransferRequest in business logic
• if all attempts failed with given request and all of them failed write transfer request into persistent storage (a separate table in a catalog)
  • if the transfer is eventually succeeded change error of transfer request to nil.
• add api which can return the list of failed transfers from aforementioned table

The api can be re-used when we'll implement the policy of "what to do with failed transfers". The table should capture all details about failed request that it can be re-initiate it at a later time.

[rishiloyola]

@vkuznet I have added code to capture the errors and to take actions according to that. Can you review this commit? Code - link