[Extensions] Pretty print for JS exceptions in the native code

Instead of just printing a warning, give the developer the line
number and possible a clue about what is going on.

Before we had:
[0926/203226:WARNING:xwalk_extension_module.cc(190)] Exception when running message listener.

Now we have:
[0926/203226:WARNING:xwalk_extension_module.cc(190)] Exception when running message listener: JS API code for xwalk.sysapps.raw_socket:430:11 ReferenceError: foobar is not defined
    foobar = 1;