output different error pages based on the REASON= in SEND_HS_DESC_FAILED_EVENT

[virgil]

see: https://trac.torproject.org/projects/tor/ticket/13212

A more informative error message would be very helpful.

From the Tor Changelog,

https://gitweb.torproject.org/tor.git/tree/ChangeLog

these are the ones:

• Refactor hidden service client-side cache lookup to intelligently
  report its various failure cases, and disentangle failure cases
  involving a lack of introduction points. Closes ticket 14391.
• When we fail to retrieve a hidden service descriptor, send the
  controller an "HS_DESC FAILED" controller event. Implements feature
  13212.

In short I want these controller events to be returned to Tor2web. It's unclear to me how to best go about this. For starters, I want to be able to distinguish among:

• Hidden service descriptor doesn't exist
• Cxn to Rendezvous Point failed
• HS descriptor exists, but can't reach introduction points
• Reached introduction points, but no response from Hidden Service.

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[fpietrosanti]

That probably require to integrate meejah's txtorcon to get back the messages based on Tor Control Port. In case you integrate it for the purpose of this ticket, please consider also implementing #203 (that require also simplifying startup script + providing a default config from code)