better error logging and corrupt jarfile messaging

If bad temp files are being copied over to your testium install, this feature is for you!

Now, whenever you try to run testium and selenium errors with `Invalid or corrupt jarfile`, you will get a better error message:

```
Error: Process "selenium" crashed. See log at: /home/sean/source/testium/test/integration_log/selenium.log.
Error: Invalid or corrupt jarfile /home/sean/source/testium/bin/selenium.jar
[TESTIUM] try ./node_modules/.bin/testium --force-update
  at procError (/home/sean/source/testium/lib/selenium/process/port.js:50:12)
  at /home/sean/source/testium/lib/selenium/process/port.js:65:19
  at Socket.<anonymous> (/home/sean/source/testium/node_modules/portscanner/lib/portscanner.js:81:5)
  at Socket.EventEmitter.emit (events.js:95:17)
  at TCP.close (net.js:465:12)
```

It points you to the full log, outputs whatever the process sent to stderr, and tells you that you can run `./node_modules/.bin/testium --force-update` to try to fix this specific issue.

To test this yourself, you can replace selenium.jar with an empty file.

```
rm ./node_modules/testium/bin/selenium.jar ; touch ./node_modules/testium/bin/selenium.jar
```
---

Depends on https://github.com/groupon-testium/selenium-download/pull/9