fix: correctly report type errors in config

[cljoly]

Issue

Currently, the types of fields in the configuration is validated when
the configuration is loaded. But the error is not reported correctly:

Traceback (most recent call last):
  File "/home/cjoly/ghq/github.com/moverest/sway-interactive-screenshot/./sway-interactive-screenshot", line 922, in <module>
    main()
    ~~~~^^
  File "/home/cjoly/ghq/github.com/moverest/sway-interactive-screenshot/./sway-interactive-screenshot", line 830, in main
    config = Config(args.config)
  File "/home/cjoly/ghq/github.com/moverest/sway-interactive-screenshot/./sway-interactive-screenshot", line 174, in __init__
    errors = list(self._validate("", self.config))
  File "/home/cjoly/ghq/github.com/moverest/sway-interactive-screenshot/./sway-interactive-screenshot", line 58, in validate
    yield from field_validators[field_name](f"{path}.{field_name}", field_value)
  File "/home/cjoly/ghq/github.com/moverest/sway-interactive-screenshot/./sway-interactive-screenshot", line 58, in validate
    yield from field_validators[field_name](f"{path}.{field_name}", field_value)
  File "/home/cjoly/ghq/github.com/moverest/sway-interactive-screenshot/./sway-interactive-screenshot", line 68, in validate
    path, f"Type {type(value)} is not one of {', '.join(types)}."
                                              ~~~~~~~~~^^^^^^^
TypeError: sequence item 0: expected str instance, type found

Here, an exception occurs when trying to instantiate ValidationError,
because the type Type is not a str, which join expects.

This MR fixes that, so we now print:

Traceback (most recent call last):
  File "/home/cjoly/ghq/github.com/moverest/sway-interactive-screenshot/./sway-interactive-screenshot", line 922, in <module>
    main()
    ~~~~^^
  File "/home/cjoly/ghq/github.com/moverest/sway-interactive-screenshot/./sway-interactive-screenshot", line 830, in main
    config = Config(args.config)
  File "/home/cjoly/ghq/github.com/moverest/sway-interactive-screenshot/./sway-interactive-screenshot", line 176, in __init__
    raise ConfigError(errors)
ConfigError: Config errors:
 .screenshot.cursor: Type <class 'str'> is not one of <class 'bool'>.

Testing

I triggered the error with the following config:

[screenshot]
cursor = "false2"