feat(component): add error as value to LetDirective's context

[markostanimirovic]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

The $error property from LetDirective's view context will be true or false.

Closes #3343

What is the new behavior?

The $error property from LetDirective's view context will be thrown error or undefined.

Does this PR introduce a breaking change?

[x] Yes
[ ] No

BREAKING CHANGES:

The `$error` property from `LetDirective`'s view context is a thrown error or `undefined` instead of `true`/`false`.

BEFORE:

<p *ngrxLet="obs$; $error as e">{{ e }}</p>

- `e` will be `true` when `obs$` emits error event.
- `e` will be `false` when `obs$` emits next/complete event.

AFTER:

<p *ngrxLet="obs$; $error as e">{{ e }}</p>

- `e` will be thrown error when `obs$` emits error event.
- `e` will be `undefined` when `obs$` emits next/complete event.

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