Correctly wait for Selendroid standalone server to come up

Summary:
`HttpClientUtil.waitForServer` sits in a loop for three minutes and then quietly exits if the server failed to start.

This fixes it:
- Throw an exception if the server didn't start.
- Shorter timeout. Starting the server is as simple as starting a thread.

Test Plan:
- `$ mvn clean install`
- Ran one the of the Facebook end-to-end tests.

Reviewers: asm

Reviewed By: asm

Subscribers: DominikDary

Differential Revision: https://reviews.facebook.net/D29739