minor #14937 Standardize the name of the exception variables (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes #14937).

Discussion
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Standardize the name of the exception variables

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

See symfony/symfony-docs#4491 for the context of this change.

In Symfony source code there are 410 `try ... catch` blocks. More than 95% of them use `$e` as the name of the exception variable. After applying these changes, 407 out of 410 variables are named `$e`.

These are the three cases where I didn't change the name of the `$e` variable:

  * Nested exception in https://github.com/symfony/symfony/blob/2.3/src/Symfony/Component/Routing/Matcher/RedirectableUrlMatcher.php#L40. It uses `$e2` as the name of the nested variable.
  * Nested exception in https://github.com/symfony/symfony/blob/2.3/src/Symfony/Bundle/TwigBundle/TwigEngine.php#L82. I changed the name of the `$ex` variable to `$e2` to match the previous syntax.
  * https://github.com/symfony/symfony/blob/2.3/src/Symfony/Component/Console/Helper/DialogHelper.php#L463. I don't know if it's safe to change the name of the `$error` exception variable.

Commits
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e8b924c Standardize the name of the exception variables

Author: fabpot

Helper/DialogHelper.php

@@ -452,18 +452,18 @@ private function hasSttyAvailable()
      */
     private function validateAttempts($interviewer, OutputInterface $output, $validator, $attempts)
     {
-        $error = null;
+        $e = null;
         while (false === $attempts || $attempts--) {
-            if (null !== $error) {
-                $output->writeln($this->getHelperSet()->get('formatter')->formatBlock($error->getMessage(), 'error'));
+            if (null !== $e) {
+                $output->writeln($this->getHelperSet()->get('formatter')->formatBlock($e->getMessage(), 'error'));
             }
 
             try {
                 return call_user_func($validator, $interviewer());
-            } catch (\Exception $error) {
+            } catch (\Exception $e) {
             }
         }
 
-        throw $error;
+        throw $e;
     }
 }