Instead, return the reason why the circuit could not be built,
as other errors do.

There is no any case in our code where path is an integer.
Remove asserts, check only the lenght of the circuit and check it
in a more logical way.

It is observed that when the circuit can not be built the first
time, consecutive attemps will also fail.
The relay will be measured anyway in the next iteration.
This will also speed up the scanner.
There is no need to remove the 3 attemps in Destination.
_perform_usability_test, since the method will be removed in #28897.

Fixes bug #29295. Bugfix v0.1.0.

Solved merge conflicts in sbws/__init__.py and sbws/core/scanner.py
after merging bug28741

that depends on the HTTP request timeout.

since the relay might start being measured some time after
apply_async is called if there're not available threads.

also split main function into an extra main_loop function to be
able to stop the threads after they have started.
Also check end event in the mean loop and before starting to
measure a new relay.

Fixes bug #28869. Bugfix v0.1.0.

so that the objects that manage the threads can be stop at any
time.

when worker thread finish.

instead of assigning it to the ResultDump object.

There was merge conflict in the imports, solved adding all of them

To easy diffs and avoid security issues.

Fixes bug #28590. Bugfix v0.4.0.

instead of bandwidth.
Forgotten when bandwidth was renamed to consensus_bandwidth in
9e089b72.

User torrc options in `extra_lines` that are only one key and not
a key value pair were not being parsed correctly.
Add a test.

Fixes bug #28715. Bugfix v0.1.1

when parsing user configuration, instead of modifying the one
passed to the function.

Fixes bug #28738. Bugfix v0.1.1

launch_tor is also parsing three sources of torrc options.
Move the `extra_lines` torrc option set by a user configuration
file to a function so that it can be tested.
Add docstring and remove comments.
Add sucess test.

Fixes bug #28738. Bugfix v0.1.1

Closes bug #28761. Bugfix v0.6.0

Fixes bug #28724. Bugfix v0.7.0.

since deb.torproject.org does not support anymore Trusty.

If the circuit no longer exists, the exception is already catched.