minor #20011 Use UUID for error codes for Form validator. (Koc)

This PR was submitted for the 2.8 branch but it was merged into the 2.7 branch instead (closes #20011).

Discussion
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Use UUID for error codes for Form validator.

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | tiny, but it allowed http://symfony.com/doc/current/contributing/code/bc.html#changing-classes
| Deprecations? | no
| Tests pass?   | I hope so :)
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Commits
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6dbb186 Use UUID for error codes for Form validator.

Author: fabpot

src/Symfony/Component/Form/Extension/Validator/Constraints/Form.php

@@ -18,14 +18,14 @@
  */
 class Form extends Constraint
 {
-    const NOT_SYNCHRONIZED_ERROR = 1;
-    const NO_SUCH_FIELD_ERROR = 2;
+    const NOT_SYNCHRONIZED_ERROR = '1dafa156-89e1-4736-b832-419c2e501fca';
+    const NO_SUCH_FIELD_ERROR = '6e5212ed-a197-4339-99aa-5654798a4854';
 
     /**
      * @deprecated since version 2.6, to be removed in 3.0.
      *             Use {@self NOT_SYNCHRONIZED_ERROR} instead.
      */
-    const ERR_INVALID = 1;
+    const ERR_INVALID = self::NOT_SYNCHRONIZED_ERROR;
 
     protected static $errorNames = array(
         self::NOT_SYNCHRONIZED_ERROR => 'NOT_SYNCHRONIZED_ERROR',