[PREVIEW] Show the possibly listening process in the AlreadyRunning exception message

[mmaslowskicc]

refs #133

What I think must be changed before it's mergable:

• we should not have the same AlreadyRunning in Executor (which is used also for PIDExecutor and could be used e.g. for Unix socket executors; these don't have port numbers): we could instead subclass it to add the port number and the new message in an exception specific to the TCPExecutor
• we should find the responsible process in the TCPExecutor, not the exception class
• since there are potential race conditions (e.g. a listening process exists between us checking if the port is available and reporting the exception message), we might want to retry the check in some cases

[coveralls]

Coverage decreased (-4.6%) to 90.173% when pulling a3f896f on mmaslowskicc:issues-133 into d86169d on ClearcodeHQ:master.

[coveralls]

Coverage decreased (-4.6%) to 90.173% when pulling a3f896f on mmaslowskicc:issues-133 into d86169d on ClearcodeHQ:master.

[coveralls]

Coverage decreased (-4.6%) to 90.173% when pulling a3f896f on mmaslowskicc:issues-133 into d86169d on ClearcodeHQ:master.

[coveralls]

Coverage decreased (-4.6%) to 90.173% when pulling a3f896f on mmaslowskicc:issues-133 into d86169d on ClearcodeHQ:master.

[coveralls]

Coverage decreased (-4.6%) to 90.173% when pulling a3f896f on mmaslowskicc:issues-133 into d86169d on ClearcodeHQ:master.

[mmaslowskicc]

Tests show also that this must be an optional feature, since psutil is not available on at least PyPy3.