Custom exceptions

Create an `./exceptions.py`.

Define any custom exceptions. Can append `Error` to the exception name if needed, otherwise be descriptive.

E.g `MismatchedTokenError`, `ScopedPageAlreadyExists`.

(If it would raise a `SystemExit(1)` use an Error).


Can set custom attributes on the Exception:

```
class MismatchedTokenError(Exception):
    def __init__(self, expected: int, given: int) -> None:
        self.expected = expected
        self.given = given
        super().__init__()
```

Can refer to these when capturing the exception:

```
except exceptions.MismatchedTokenError as exec_info:
    click.echo(exec_info.expected, exec_info.given)
    # error handling
```

Alternatively you can pass arbitrary `*args` into the exceptions and capture them with `args[0]`:

```
raise exceptions.TemplateNotFoundError("some message")
```

```
except exceptions.TemplateFileNotFoundError as exec_info:
    cli_message(exec_info.args[0])
```