custom-error-definition - Fix null reference

[MrHen]

Fixes #150.

I did NOT try to verify that the name used was correct (e.g. export.FooError versus export.foo). I just fixed the null ref.

As of this PR, these cases will all pass:

exports.fooError = class extends Error {
    constructor(error) {
        super(error);
    }
};

exports.fooError = class extends Error {
    constructor(error) {
        super(error);
        this.name = 'fooError';
    }
};

exports.fooError = class FooError extends Error {
    constructor(error) {
        super(error);
        this.name = 'FooError';
    }
};

[sindresorhus]

Can you add this as a test?

exports.fooError = class extends Error {
    constructor(error) {
        super(error);
        this.name = 'fooError';
    }
};

[sindresorhus]

Can you add this as a commented out failing test and open an issue about it?

exports.fooError = class FooError extends Error {
    constructor(error) {
        super(error);
        this.name = 'FooError';
    }
};

[sindresorhus]

Thanks :)