Hyperbahn client hides error when failing to read hostPortFile

[Willyham]

The only thing reported is:

{"log":"AssertionError: Failed to read the hostPortFile\n","stream":"stderr","time":"2015-09-04T18:04:31.825560731Z"}

Which isn't very helpful.

[Raynos]

👍 We should fix this.

[Raynos]

cc @ShanniLi