MAGETWO-31904: There is no validation for 'Table prefix' on Step 2: Add a Database durin

- Fixed messages (approved from Daniel)
- Fixed method comment.

Author: Ivan Gavryshko

setup/src/Magento/Setup/Model/Installer.php

@@ -758,7 +758,7 @@ public function checkDatabaseConnection($dbName, $dbHost, $dbUser, $dbPass = '')
     }
 
     /**
-     * Check if database table is valid
+     * Check if database table prefix is valid
      *
      * @param string $prefix
      * @return boolean
@@ -767,7 +767,7 @@ public function checkDatabaseConnection($dbName, $dbHost, $dbUser, $dbPass = '')
     public function checkDatabaseTablePrefix($prefix)
     {
         if ($prefix !== '' && !preg_match('/^([[:alnum:]]+)([[:alnum:]_]+)$/', $prefix)) {
-            throw new \InvalidArgumentException('Table prefix is in wrong format.');
+            throw new \InvalidArgumentException('Please correct the table prefix format.');
         }
 
         return true;