Improve error messages for PHABRICATOR_ENV

Summary:

• Allow user to specify "myconf" (recommended) or "myconf.conf.php" (less

surprising).

• Make sure syntax errors and other problems are surfaced.
• If the configuration value isn't valid, give them a list of all valid

values.

Test Plan:

• Added a syntax error, got a useful error.
  • Set PHABRICATOR_ENV to a silly value, got a list of valid values.
  • Set PHABRICATOR_ENV to have .conf.php suffix, site still worked.

Reviewed By: kevinwallace
Reviewers: kevinwallace, codeblock, aran, jungejason, tuomaspelkonen
CC: aran, epriestley, kevinwallace
Differential Revision: 381