When my mac crashes and Postgres doesn't boot properly I see the following errors in /usr/local/var/postgres/server.log
FATAL: lock file "postmaster.pid" already exists
# and / or
lock file "/var/pgsql_socket/.s.PGSQL.5432.lock" already exists
Here are the commands to put Postgres back on track:
launchctl unload ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist
rm /var/pgsql_socket/.s.PGSQL.5432.lock
rm /usr/local/var/postgres/postmaster.pid
launchctl load ~/Library/LaunchAgents/homebrew.mxcl.postgresql.plist