[Process] fixed fatal errors in getOutput and getErrorOutput when process was not started

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

Added logic exceptions for prevent fatal errors in ``getOutput`` and ``getErrorOutput`` when process was not started.

```
Fatal error: Call to a member function readAndCloseHandles() on a non-object in /home/max/symfony/src/Symfony/Component/Process/Process.php on line 1117

Call Stack:
    0.0002     231816   1. {main}() /home/max/phpunit/composer/bin/phpunit:0
    0.0038     526688   2. PHPUnit_TextUI_Command::main() /home/max/phpunit/composer/bin/phpunit:63
    0.0038     526912   3. PHPUnit_TextUI_Command->run() /home/max/phpunit/PHPUnit/TextUI/Command.php:126
    0.2261    4022696   4. PHPUnit_TextUI_TestRunner->doRun() /home/max/phpunit/PHPUnit/TextUI/Command.php:173
    0.2302    4353544   5. PHPUnit_Framework_TestSuite->run() /home/max/phpunit/PHPUnit/TextUI/TestRunner.php:415
    0.3218    5675504   6. PHPUnit_Framework_TestSuite->run() /home/max/phpunit/PHPUnit/Framework/TestSuite.php:729
   15.1171   38257632   7. PHPUnit_Framework_TestCase->run() /home/max/phpunit/PHPUnit/Framework/TestSuite.php:729
   15.1172   38257632   8. PHPUnit_Framework_TestResult->run() /home/max/phpunit/PHPUnit/Framework/TestCase.php:782
   15.1173   38258544   9. PHPUnit_Framework_TestCase->runBare() /home/max/phpunit/PHPUnit/Framework/TestResult.php:661
   15.1174   38275544  10. PHPUnit_Framework_TestCase->runTest() /home/max/phpunit/PHPUnit/Framework/TestCase.php:839
   15.1175   38276408  11. ReflectionMethod->invokeArgs() /home/max/phpunit/PHPUnit/Framework/TestCase.php:987
   15.1175   38276440  12. Symfony\Component\Process\Tests\AbstractProcessTest->testGetOutputProcessNotRunning() /home/max/phpunit/PHPUnit/Framework/TestCase.php:987
   15.1175   38277728  13. Symfony\Component\Process\Process->getOutput() /home/max/symfony/src/Symfony/Component/Process/Tests/AbstractProcessTest.php:636
   15.1175   38277824  14. Symfony\Component\Process\Process->readPipes() /home/max/symfony/src/Symfony/Component/Process/Process.php:383
```

- [x] fix code after review by @Tobion
- [ ] fix similar problems