Fix an issue where "arc" may fail on startup when trying to read older "default" config
Fix an issue where "arc" may fail on startup when trying to read older "default" config

Summary:
See PHI1663. If "phabricator.uri" is not configured, we try to fall back to "default", but doesn't have a modern config specification and fails.

Instead, read "default" as a raw config value to preserve compatible behavior. My intent is to eventually remove it.

Test Plan:
In a directory with no "phabricator.uri" config available, ran `echo {} | arc call-conduit conduit.ping`. After the patch, got a reasonable error instead of a fatal.

In a directory with "default" configured but not "phabricator.uri", ran the same command. After the patch, got a ping.

Maniphest Tasks: T13497

Differential Revision: https://secure.phabricator.com/D21039

codelabs-ch/arcanist,phacility/arcanist,Khan/arcanist,phacility/arcanist,Khan/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,phacility/arcanist,phacility/arcanist,Khan/arcanist,phacility/arcanist,codelabs-ch/arcanist,codelabs-ch/arcanist,Khan/arcanist