… into UUIDs (webmozart)

This PR was merged into the 2.8 branch.

Discussion
----------

[Validator] Added missing error codes and turned codes into UUIDs

Reopened #12388 on the 2.8 branch.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

With the current implementation of error codes, checking which error occurred is unnecessarily complex:

```php
if ($violation->getConstraint() instanceof Length && Length::TOO_SHORT_ERROR === $violation->getCode()) {
    // ...
}
```

Also, the code is completely missing for some constraints. This is fixed now. By using UUIDs, the check is reduced to:

```php
if (Length::TOO_SHORT_ERROR === $violation->getCode()) {
    // ...
}
```

Also, APIs can simply output the error code and the name of the error without needing to point to the constraint as well.

Before:

```json
[
    {
        "code": "1",
        "name": "TOO_SHORT_ERROR",
        "message": "This value is too short. ...",
        "constraint": "Symfony\\Component\\Validator\\Constraints\\Length"
    }
]
```

After:

```json
[
    {
        "code": "9ff3fdc4-b214-49db-8718-39c315e33d45",
        "name": "TOO_SHORT_ERROR",
        "message": "This value is too short. ..."
    }
]
```

This makes it possible to implement a service on symfony.com which looks up error codes, e.g.

symfony.com/error?code=9ff3fdc4-b214-49db-8718-39c315e33d45

Such a URL could redirect directly to the documentation of the appropriate constraint. We could even support user-submitted error codes which redirect to the documentation of that constraint.

Commits
-------

8874e88 [Validator] Added missing error codes and turned codes into UUIDs

…(Koc)

This PR was merged into the 2.8 branch.

Discussion
----------

[DoctrineBridge] added missing error code for constraint.

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | wait for travis
| Fixed tickets | #15154
| License       | MIT
| Doc PR        | -

Commits
-------

32cb269 [DoctrineBridge] added missing error code for constraint.

…straint fails. (Koc)

This PR was merged into the 3.3-dev branch.

Discussion
----------

[DX][Form][Validator] Add ability check if cocrete constraint fails.

| Q | A |
| --- | --- |
| Branch? | master |
| Bug fix? | no |
| New feature? | yes |
| BC breaks? | no |
| Deprecations? | no |
| Tests pass? | wait for travis |
| Fixed tickets | #15154 |
| License | MIT |
| Doc PR | should open |

Sometimes for big forms with multiple constraints we should handle some errors separately.

``` php
// when using validator
$constraintViolations = $validator->validate(...);
if (count($constraintViolations->findByCodes(UniqueEntity::NOT_UNIQUE_ERROR))) {
  // display some message or send email or etc
}

// when using forms
if (count($form->getErrors()->findByCodes(UniqueEntity::NOT_UNIQUE_ERROR))) {
  // display some message or send email or etc
}
```

This PR add some useful methods to handle this. Before we should iterate all failed constraints using foreach.

Feel free to suggest better names for new methods.

Commits
-------

29a3a7e Add ability retrieve errors by their code.