Improve error handling (#33)

* refactor: improve error handling

* fix: specify bind address instead of port

* fix: remove temporary testing file

* fix(errors): change error names to snake_case

* refactor(errors): split indexing error types, remove unused errors

* feat: add env variable checking at program start

This just checks whether the enviroment variables exist and can
parse to the given type and gives a warning if they can't. This
should prevent cases where the program fails at runtime due to
checking an environment variable that doesn't exist.

.env

@@ -4,7 +4,7 @@ DEBUG=true
 MONGODB_ADDR=mongodb://localhost:27017
 MEILISEARCH_ADDR=http://localhost:7700
 
-PORT=8000
+BIND_ADDR=127.0.0.1:8000
 
 BACKBLAZE_KEY_ID=none
 BACKBLAZE_KEY=none