Detach from debugger when front-end disconnects.

Modified session behaviour so that back-end closes session and detaches
from the debugger when front-end web-socked disconnects.

Changed handling of debugger-connection errors so that front-end displays
popup window with error description when back-end cannot connect to debugger.

When a session starts, all previously set breakpoints are removed, since
they were already deactivated by V8 debugger.
Fixed title of html page and removed null error

Changed page title string to 'Node Inspector' (with no parameters).

Added override for _calculateWorkerInspectorTitle to prevent
null error being reported to front-end console window.