/**
Assert that the given value is an `Error`.

If the value is not an `Error`, a helpful `TypeError` will be thrown.

This can be useful as any value could potentially be thrown, but in practice, it's always an `Error`. However, because of this, TypeScript makes the caught error in a try/catch statement `unknown`, which is inconvenient to deal with.

@param message - Custom error message to use instead of the default.

@example
```
import {assertError} from 'ts-extras';

try {
    fetchUnicorns();
} catch (error: unknown) {
    assertError(error);

    // `error` is now of type `Error`

    if (error.message === 'Failed to fetch') {
        retry();
        return;
    }

    throw error;
}

assertError('not an error', 'Expected an Error instance');
//=> TypeError: Expected an Error instance
```

@category Type guard
*/
export declare function assertError(value: unknown, message?: string): asserts value is Error;