MAGETWO-64226: Add paths validation

Author: shiftedreality

app/code/Magento/Config/Model/Config/PathValidator.php

@@ -39,7 +39,7 @@ public function validate($path)
         $allPaths = $this->structure->getFieldPaths();
 
         if (!array_key_exists($path, $allPaths)) {
-            throw new ValidatorException(__('The "%1" path does not exists', $path));
+            throw new ValidatorException(__('The "%1" path does not exist', $path));
         }
 
         return true;

app/code/Magento/Config/Test/Unit/Model/Config/PathValidatorTest.php

@@ -55,7 +55,7 @@ public function testValidate()
 
     /**
      * @expectedException \Magento\Framework\Exception\ValidatorException
-     * @expectedExceptionMessage The "test/test/test" path does not exists
+     * @expectedExceptionMessage The "test/test/test" path does not exist
      */
     public function testValidateWithException()
     {

dev/tests/integration/testsuite/Magento/Config/Console/Command/ConfigSetCommandTest.php

@@ -521,7 +521,7 @@ function (Mock $output) {
                 function (Mock $output) {
                     $output->expects($this->once())
                         ->method('writeln')
-                        ->with('<error>The "test/test/test" path does not exists</error>');
+                        ->with('<error>The "test/test/test" path does not exist</error>');
                 },
                 'test/test/test',
                 'value',