errors: Maintain caller location in to_log

[har7an]

The errors::LoggableError trait provides the to_log method as convenience function to log error messages easily. It is increasingly used throughout the zellij codebase, either directly or indirectly from the non_fatal function.

Previously, the log messages generated by to_log would look something like this:

ERROR  |zellij_utils::errors::not| 2022-12-05 21:22:07.662 [stdin_handler] [zellij-utils/src/errors.rs:130]:  a non-fatal error occured

Caused by:
[ ... ]

The log output tells us that the error occured at zellij-utils/src/errors.rs:130, which is in fact wrong: This is only the location where to_log calls log::error! to log the actual message content. It loses information about where the error originated.

This PR changes the way the messages are logged, and provides log entries like the following instead:

ERROR  |???                      | 2022-12-06 10:04:57.527 [screen    ] [zellij-server/src/screen.rs:1931]: a non-fatal error occured

Caused by:
[ ... ]

Most notably, now the file and line number of the real caller are preserved. Unfortunately, this leaves the module empty ("???"), but this is unavoidable (and wasn't correct previously, either). That is because log by default resolves the module name with the std::module_path!() macro, which is replaced at compile time in the location it is written. In our case, this would still be the to_log function. Hence, we leave the module empty.