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:

tool-sync/src/config/toml.rs

Lines 82 to 86 in a64ab97

	fn parse_file(config_path: &PathBuf, proxy: Option<String>) -> Result<Config, TomlError> {
	let contents = fs::read_to_string(config_path).map_err(|e| TomlError::IO(format!("{}", e)))?;
	parse_string(&contents, proxy)
	}