R1:7aa35821201a

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…