feat(python): catch all exceptions thrown through fastapi route paths

This commit does quite a bit:
- Catches unhandled exceptions raised in the route handler and
  produces a 500 Internal Server Error Arch-themed response.
- Each unhandled exception causes a notification to be sent to new
  `notifications.postmaster` email with a "Traceback ID."
- Traceback ID is logged to the server along with the traceback which
  caused the 500: `docker-compose logs fastapi | grep '<traceback_id>'`
- If `options.traceback` is set to `1`, traceback is displayed in
  the new 500.html template.

Signed-off-by: Kevin Morris <kevr@0cost.org>

conf/config.defaults

@@ -44,6 +44,8 @@ cache_pkginfo_ttl = 86400
 memcache_servers = 127.0.0.1:11211
 salt_rounds = 12
 redis_address = redis://localhost
+; Toggles traceback display in templates/errors/500.html.
+traceback = 0
 
 [ratelimit]
 request_limit = 4000
@@ -65,6 +67,9 @@ smtp-user =
 smtp-password =
 sender = notify@aur.archlinux.org
 reply-to = noreply@aur.archlinux.org
+; Administration email which will receive notifications about
+; various server details like uncaught exceptions.
+postmaster = admin@example.org
 
 [fingerprints]
 Ed25519 = SHA256:HQ03dn6EasJHNDlt51KpQpFkT3yBX83x7BoIkA1iv2k

conf/config.dev

@@ -37,6 +37,7 @@ memcache_servers = memcached:11211
 ; If cache = 'redis' this address is used to connect to Redis.
 redis_address = redis://127.0.0.1
 aur_request_ml = aur-requests@localhost
+traceback = 1
 
 [notifications]
 ; For development/testing, use /usr/bin/sendmail