getConfigEnv fails fast when key is not found and no default value is given.

Summary:
T2345
getConfig throws an Exception when the key does not exist.
Also removes dead code that throws an Exception.

Test Plan:
Reloaded the Phabricator home page.  In the process, found
2 Exceptions thrown due to nonexistent keys.  After addressing these problems,
the home page loads without Exceptions.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4541