L2C5: Fix #66 WebDriverFirefoxLinuxCrash fails on linux

We are group L2C5 from Prof. Mesbah's course

We found three types of error while fixing #66 :
1. killall firefox/bin should not include bin
2. expected exception check should be called before the exception will happen
3. some function calls only return an object, no action is actually carried (eg. navigate() returns Navigate object, should be navigate().back())
4 changed webdriverexception to unreachablebrowserexception because we think the unreachable browser exception is a more specific exception