Handle error when SharedQueue's mutex lock fails to be acquired

[yousefmoazzam]

Handling the error rather than panicking probably would be more sensible:

bittorrent-client/src/work.rs

Lines 33 to 43 in e85a8f2

	/// Get work element from front of queue
	pub fn dequeue(&self) -> Option<Work> {
	let mut lock = self.inner.lock().unwrap();
	lock.0.pop_front()
	}
	/// Put work element onto front of queue
	pub fn enqueue(&self, work: Work) {
	let mut lock = self.inner.lock().unwrap();
	lock.0.push_front(work);
	}