Rethrow common server exceptions with better messages

[Chrezm]

Describe the current situation: Server owners who accidentally forget to install requirements, do not rename config_sample to config or are missing some other files will get angry Python messages complaining about them, with no sort of indication of what to do to solve them.
Describe your feature proposal: The server throws more descriptive error messages when very common mistakes are made when launching a server. For example, if the server host did not install the YAML library, the server will make it very explicit what sort of steps to take (namely install requirements.txt). Similarly, if a required YAML file or Python file is missing, it will try its best to make it very explicit what sort of steps to take (like rename a folder or get a fresh download).
Describe relevant existing functionality, if any: Currently, the server throws Python errors on console and shuts down.