Output a helpful message when there's no configuration

[chshersh]

Currently, when you run tool sync and there's no .tool.toml file, you see the following message:

❯ tool sync
Aborting 'tool-sync' with error:

    * Error parsing configuration at path /home/chshersh/.tool.toml: [IO Error] No such file or directory (os error 2)

Instead, it would be better to tell more information:

• [ ] How to generate the default configuration file (copy instructions from the documentation)
• [ ] Tell, how you can specify the path of a custom configuration using the --config flag

This would require matching on error here:

https://github.com/chshersh/tool-sync/blob/a64ab97025f635472e8526ef362e6dc57752fd52/src/config/toml.rs#L82-L86