Throw RuntimeException when Phabricator API call fails.

Summary:
The Phabricator API is called through curl. If the session fails, we
now throw an exception instead to return false in Phabricator::post.

Test Plan:
Call config:show command in unit test context.

An exception should be thrown, as phabricator.acme.tld shouldn't succeed.

Formal tests to come in D241.

Reviewers: dereckson

Subscribers: alken-orin

Differential Revision: http://devcentral.nasqueron.org/D315